I totally see your point in terms of value, but rockbottom is really only buffing the deck guns, which aren't good, and Macnaile is buffing the galleon? It's more free points, for sure, but I don't really think you're getting the value out of 2 deck guns, rockbottom, and macnaile that you get out of the DSM.
They contest like a boss and have recursion. Without them, your only units are your cannon, deck guns, and A&H, none of which you want to advance enough to take a zone. That being said, I've really only got Meg for flags, so YMMV. I think I'm gonna try to kitbash some swabbers to try it out one way or the other.
I'm a sucker for drag (wow, that's a sentence) and shield guard + girded + lash is a fantastic kit for supporting a high def warcaster (Ashlynn or Rahera with black spot) and powder monkey plus swabber opens up a ton of wonky shenanigans. It seems like a ton of fun with lots of attack vectors. It also has a surprising amount of punch with the ability to generate extra attacks.