Being ARM 18 vs 20 on heavy, and ARM 15 vs 17 on unit are totally different story, especially on DEF 12 models.
Also you can't directly compare Warmachine Faction with Hordes faction - you know the fury tax, that is not only make Hordes stuffs expensive but also gives the better statline over the Warmachine factions.
And you can't compare SPD 4 Juggernaut wit SPD 5 Dire Trolls, seriously, although Dire Troll's defensive stats and speed are same as Cygnar Ironclad chassis(which is infamous for its pitiful durability on its own).
The point of defensive stat is how much it can soak the damage before fell off. Honestly, every single thing can be killed in the game. But the point is how easy to do so.
For heavy, ARM 18 is nothing but a laugh. That's barely able to withstand anti-armor ranged weapons in the game(that is usually boosted POW 14 shot or something similar). In melee, they are so easily melt down.
Units are just same. ARM 15 is not meant to survive against anything save for non-High Explosive blasts, but ARM 17 can have some possibility to survive against POW 10 shots. In melee they are easily dying out, but isn't melee attack supposed to be kill the units in the first place?
Let me show the probability how easily they will fell without the stone.
Case 1-1: A MAT 6 opponent makes an attack with POW 18 melee weapon against 12/18 Dire Trolls(with 30 points of damage spiral).
-With Stone: 72.22..% to hit(6+), average 5.0 points of damage -> 3.611.. average damage per an attack - requires about 8.3 attacks to kill it.
-Without it: 72.22..% to hit(6+), average 7.0 points of damage -> 5.055.. average damage per an attack - requires about 5.93 attacks to kill it.
Case 1-1-1: A MAT 7 opponent makes an attack with POW 18 melee weapon against 12/18 Dire Trolls(with 30 points of damage spiral).
-With Stone: 83.33..% to hit(5+), average 5.0 points of damage -> 4.166.. average damage per an attack - requires about 7.2 attacks to kill it.
-Without it: 83.33..% to hit(5+), average 7.0 points of damage -> 5.833.. average damage per an attack - requires about 5.14 attacks to kill it.
Case 1-1-2: A
MAT 7 opponent makes an attack with
POW 19 melee weapon against
12/18 Dire Trolls(with 30 points of damage spiral).
-With Stone: 83.33..% to hit(5+), average 6.0 points of damage -> 5.0 average damage per an attack - requires about 6 attacks to kill it.
-Without it: 83.33..% to hit(5+), average 8.0 points of damage -> 6.66.. average damage per an attack - requires about 4.50 attacks to kill it.
Case 1-1-3: A
MAT 6 opponent makes an attack with
POW 20 melee weapon against
12/18 Dire Trolls(with 30 points of damage spiral).
-With Stone: 72.22..% to hit(6+), average 7.0 points of damage -> 5.055.. average damage per an attack - requires about 5.93 attacks to kill it.
-Without it: 72.22..% to hit(6+), average 9.0 points of damage -> 6.5 average damage per an attack - requires about 4.61 attacks to kill it.
Case 1-2: A
RAT 5 opponent makes an attack with
POW 14 ranged weapon with boosted attack and damage roll against
12/18 Dire Trolls(with 30 points of damage spiral).
-With Stone: 90.74..% to hit(7+), average 4.569.. points of damage -> 4.146.. average damage per an attack - requires about 7.23 attacks to kill it.
-Without it: 90.74..% to hit(7+), average 6.504.. points of damage -> 5.902.. average damage per an attack - requires about 5.08 attacks to kill it.
Case 1-2-1: A
RAT 6 opponent makes an attack with
POW 15 ranged weapon with boosted attack and damage roll against
12/18 Dire Trolls(with 30 points of damage spiral).
-With Stone: 95.37..% to hit(6+), average 5.523.. points of damage -> 5.267.. average damage per an attack - requires about 5.69 attacks to kill it.
-Without it: 95.37..% to hit(6+), average 7.5 points of damage -> 7.152.. average damage per an attack - requires about 4.19 attacks to kill it.
Case 1-2-2: A
RAT 7 opponent makes an attack with
Armor Piercing POW 6 ranged weapon and boosted attack and damage roll against
12/18 Dire Trolls(with 30 points of damage spiral).
-With Stone: 98.148..% to hit(5+), average 5.523.. points of damage -> 5.42 average damage per an attack - requires about 5.53 attacks to kill it.
-Without it: 98.148..% to hit(5+), average 7.5 points of damage -> 7.361.. average damage per an attack - requires about 4.07 attacks to kill it.
Case 1-2-3: A
RAT 5 opponent makes an attack with
POW 16 ranged weapon with boosted attack and damage roll against
12/18 Dire Trolls(with 30 points of damage spiral).
-With Stone: 90.74..% to hit(7+), average 6.504.. points of damage -> 5.902.. average damage per an attack - requires about 5.08 attacks to kill it.
-Without it: 90.74..% to hit(7+), average 8.5 points of damage -> 7.712.. average damage per an attack - requires about 3.89 attacks to kill it.
Case 1-3: A
RAT 7 opponent makes an attack with
POW 12 ranged weapon against
12/18 Dire Trolls(with 30 points of damage spiral).
-With Stone: 83.33..% to hit(5+), average 0.555.. points of damage -> 0.462.. average damage per an attack - requires about 64.93 attacks to kill it.
-Without it: 83.33..% to hit(5+), average 1.555.. points of damage -> 1.296.. average damage per an attack - requires about 23.14 attacks to kill it.
Case 1-3-1: A
RAT 7 opponent makes an attack with
POW 13 ranged weapon against
12/18 Dire Trolls(with 30 points of damage spiral).
-With Stone: 83.33..% to hit(5+), average 0.972.. points of damage -> 0.81 average damage per an attack - requires about 37.03 attacks to kill it.
-Without it: 83.33..% to hit(5+), average 2.277 points of damage -> 1.898.. average damage per an attack - requires about 15.80 attacks to kill it.
Case 1-3-2: A
RAT 7 opponent makes an attack with
POW 14 ranged weapon against
12/18 Dire Trolls(with 30 points of damage spiral).
-With Stone: 83.33..% to hit(5+), average 1.555.. points of damage -> 1.296.. average damage per an attack - requires about 23.14 attacks to kill it.
-Without it: 83.33..% to hit(5+), average 3.111 points of damage -> 2.592.. average damage per an attack - requires about 7.71 attacks to kill it.
Case 1-3-3: A
RAT 9 opponent makes an attack with
POW 15 ranged weapon against
12/18 Dire Trolls(with 30 points of damage spiral).
-With Stone: 97.22..% to hit(3+), average 2.277.. points of damage -> 2.214.. average damage per an attack - requires about 13.55 attacks to kill it.
-Without it: 97.22..% to hit(3+), average 4.027 points of damage -> 3.915.. average damage per an attack - requires about 7.66 attacks to kill it.
Case 1-3-4: A
RAT 16 opponent makes an attack with
POW 20 ranged weapon against
12/18 Dire Trolls(with 30 points of damage spiral).
-With Stone: 97.22..% to hit(3+), average 7.0 points of damage -> 6.805.. average damage per an attack - requires about 4.40 attacks to kill it.
-Without it: 97.22..% to hit(3+), average 9.0 points of damage -> 8.75 average damage per an attack - requires about 3.42 attacks to kill it.
Case 2-1: A RAT 7 opponent makes an attack with POW 12 ranged weapon against 12/16 Trollkin Champions(able to sustain up to 20 damage without any loss).
-With Stone: 83.33..% to hit(5+), average 1.555.. points of damage -> 1.296.. average damage per an attack - soaks around 15.43 attacks without loss.
-Without it: 83.33..% to hit(5+), average 3.111.. points of damage -> 2.592.. average damage per an attack - soaks around 7.71 attacks without loss.
Case 2-1-1: A RAT 7 opponent makes an attack with POW 13 ranged weapon against 12/16 Trollkin Champions(able to sustain up to 20 damage without any loss).
-With Stone: 83.33..% to hit(5+), average 2.277.. points of damage -> 1.898.. average damage per an attack - soaks around 10.53 attacks without loss.
-Without it: 83.33..% to hit(5+), average 4.027.. points of damage -> 3.356.. average damage per an attack - soaks around 5.959 attacks without loss.
Case 2-1-2: A RAT 7 opponent makes an attack with POW 14 ranged weapon against 12/16 Trollkin Champions(able to sustain up to 20 damage without any loss).
-With Stone: 83.33..% to hit(5+), average 3.111.. points of damage -> 2.592.. average damage per an attack - soaks around 7.71 attacks without loss.
-Without it: 83.33..% to hit(5+), average 5.0 points of damage -> 4.166.. average damage per an attack - soaks around 4.80 attacks without loss.
Case 2-1-3: A RAT 9 opponent makes an attack with POW 15 ranged weapon against 12/16 Trollkin Champions(able to sustain up to 20 damage without any loss).
-With Stone: 97.22..% to hit(3+), average 4.027.. points of damage -> 3.915.. average damage per an attack - soaks around 5.10 attacks without loss.
-Without it: 97.22..% to hit(3+), average 6.0 points of damage -> 5.833.. average damage per an attack - soaks around 3.42 attacks without loss.
Case 2-2: A
RAT 5 opponent makes an attack with
POW 14 ranged weapon with boosted attack and damage roll against
12/16 Trollkin Champions(able to sustain up to 20 damage without any loss).
-With Stone:: 90.74..% to hit(7+), average 6.504.. points of damage -> 5.902.. average damage per an attack - soaks around 3.38 attacks without loss.
-Without it: 90.74..% to hit(7+), average 8.5 points of damage -> 7.712.. average damage per an attack - soaks around 2.59 attacks without loss.[
Case 2-2-1: A
RAT 6 opponent makes an attack with
POW 15 ranged weapon with boosted attack and damage roll against
12/16 Trollkin Champions(able to sustain up to 20 damage without any loss).
-With Stone: 95.37..% to hit(6+), average 7.5 points of damage -> 7.152.. average damage per an attack - soaks around 2.79 attacks without loss.
-Without it: 95.37..% to hit(6+), average 9.5 points of damage -> 9.06.. average damage per an attack - soaks around 2.20 attacks without loss.
Case 2-2-2: A
RAT 7 opponent makes an attack with
Armor Piercing POW 6 ranged weapon and boosted attack and damage roll against
12/16 Trollkin Champions(able to sustain up to 20 damage without any loss).
-With Stone: 98.148..% to hit(5+), average 6.504.. points of damage -> 6.384.. average damage per an attack - soaks around 3.13 attacks without loss.
-Without it: 98.148..% to hit(5+), average 8.5 points of damage -> 8.342.. average damage per an attack - soaks around 2.39 attacks without loss.
Case 2-2-3: A
RAT 5 opponent makes an attack with
POW 16 ranged weapon with boosted attack and damage roll against
12/16 Trollkin Champions(able to sustain up to 20 damage without any loss).
-With Stone: 90.74..% to hit(7+), average 8.5 points of damage -> 7.712.. average damage per an attack - soaks around 2.59 attacks without loss.
-Without it: 90.74..% to hit(7+), average 10.5 points of damage -> 9.527.. average damage per an attack - soaks around 2.099 attacks without loss.
Case 3-1: A
RAT 5 opponent makes an attack with
POW 10 ranged weapon against
12/14 Trollkin Fennblades(with Tough).
-With Stone: 58.33..% to hit(7+), 38.888% to box(7+) -> 22.685% to destroy.
-Without it: 58.33..% to hit(7+), 55.555% to box(5+) -> 32.407% to destroy.
Case 3-1-1: A
RAT 6 opponent makes an attack with
POW 10 ranged weapon against
12/14 Trollkin Fennblades(with Tough).
-With Stone: 72.222..% to hit(6+), 38.888% to box(7+) -> 28.086% to destroy.
-Without it: 72.222..% to hit(6+), 55.555% to box(5+) -> 40.123% to destroy.
Case 3-1-2: A
RAT 7 opponent makes an attack with
POW 10 ranged weapon against
12/14 Trollkin Fennblades(with Tough).
-With Stone: 83.33..% to hit(5+), 38.888% to box(7+) -> 32.407% to destroy.
-Without it: 83.33..% to hit(5+), 55.555% to box(5+) -> 46.296% to destroy.
Case 3-1-3: A
RAT 5 opponent makes an attack with
POW 11 ranged weapon against
12/14 Trollkin Fennblades(with Tough).
-With Stone: 58.33..% to hit(7+), 48.148% to box(6+) -> 28.086% to destroy.
-Without it: 58.33..% to hit(7+), 61.111% to box(4+) -> 35.648% to destroy.
Case 3-1-4: A
RAT 6 opponent makes an attack with
POW 11 ranged weapon against
12/14 Trollkin Fennblades(with Tough).
-With Stone: 72.222..% to hit(6+), 48.148% to box(6+) -> 34.773% to destroy.
-Without it: 72.222..% to hit(6+), 61.111% to box(4+) -> 44.135% to destroy.
Case 3-1-5: A
RAT 7 opponent makes an attack with
POW 11 ranged weapon against
12/14 Trollkin Fennblades(with Tough).
-With Stone: 83.33..% to hit(5+), 48.148% to box(6+) -> 40.123% to destroy.
-Without it: 83.33..% to hit(5+), 61.111% to box(4+) -> 50.925% to destroy.
Case 3-1-6: A
RAT 5 opponent makes an attack with
POW 12 ranged weapon against
12/14 Trollkin Fennblades(with Tough).
-With Stone: 58.33..% to hit(7+), 55.555% to box(5+) -> 32.407% to destroy.
-Without it: 58.33..% to hit(7+), 64.814% to box(3+) -> 37.808% to destroy.
Case 3-1-7: A
RAT 7 opponent makes an attack with
POW 14 ranged weapon against
12/14 Trollkin Fennblades(with Tough).
-With Stone: 83.33..% to hit(5+), 64.814% to box(3+) -> 54.012% to destroy.
-Without it: 83.33..% to hit(5+), 66.666% to box(2+) -> 55.555% to destroy.
Case 3-1-8: A
RAT 9 opponent makes an attack with
POW 15 ranged weapon against
12/14 Trollkin Fennblades(with Tough).
-With Stone: 97.22..% to hit(3+), 66.666% to box(2+) -> 64.814% to destroy.
-Without it: 97.22..% to hit(3+), 66.666% to box(2+) -> 64.814% to destroy.
Box rate includes
Tough. For example, if a damage roll disables the model by 100%, Tough kicks in and its rate of box become 66.666..%
On most result, +2 to ARM buff makes them sustain much longer, obviously. Only the most ridiculously high powered shots(such as Dire Troll Bomber or a CRA of Idrian Skirmisher+Prey) are able to crush the difference, but the models with damage box are still sustain longer when it has the stone.
Without the stone, Dire Trolls are even threatened by units with infantry rifles.
Even for ARM 14+2 shows that it is at least better than ARM 14, although I think that ARM below of 17 is not so meaningful for the units without damage box.
And, since the stone protects most of your forces at once, rather than only a specific model/unit. Then, why you are bothered to drop the stone?
Honestly, I think that the stone is a design fault, for Trollbloods are required to be near of it and it so good at manauver.