Its main use case is escorting larger ships, screening in front of them and pinning down light fighter cover (via AA mode), light frigate-sized Warships or other pocket warships, while the allied Warship presumably engages things that are bigger and more important. It's just one example of a Pocket Warship, wouldn't be too difficult to have another adjust some attributes here and there.
Yeah, the vanilla rules make other stuff more efficient. The optimal option for a gunboat would have been to use about 70 Clan LPLs instead of the Subcaps, and use the spare weight for more speed, probably... but that's dumb. And with Aerospace especially I have to intentionally make suboptimal decisions on paper, lest I end up with abominations like a fast 2,000 LPL "Warship" that wipes everything lighter than a Leviathan in one turn, with some LAMS to nullify any Capital Missiles that get a bearing.
(That's only funny to fight once, btw)
Non-capital fighter weapons inflicting damage to capital-scale armor is one of the things of the Aerospace-level game that I'm... less than enthusiastic about. I usually take stuff like C-Bill costs and whatnot more as general indicators than the absolute truth, but that goes triple for Aerospace.