The point is that it can be satisfying to make a thing "yours" through personalized touches. The ability to dedicate a ship to its role & christen it as "completed" by building in its primary guns so that its role can never be changed once you've scraped together all the parts it needs to be exactly what you've wanted it to become would give the game a completionist-specific kind of fun. Building in a gun could do nothing & it still cost full OP to have it there and I'd still think it's a good idea to add it to the game.
I just don't feel that the mass produced guns getting built-in would add any sense of personalization to your ships either. [HIGHLY REDACTED] weapons and other rarities, maaaaaybe. Like if you could find special guns that have degraded performance unless permanently fused to the ship hull. That would give your ship a sense of "oh, it's THAT ship". Or finding/capturing ships with rare weapons built in. But not just some Common weapon.
Though maybe it could also be a drawback for some heavily d-modded ships, having some weapons be stuck inside their mounts. I could see pirates doing that to a gun that was falling apart...
Mass produced or a one off super weapon, the fact someone is taking the time and effort to meld the gun and the ship's hull makes it a note-worthy custom job. I mean, there are modern cars today that are popular, famous, and impressive and in the end, it's basically just body work that doesn't touch the frame and probably not even mounting brackets. And considering it removes the weapon slot from the hull by permanently occupying it, I say it's a more serious consideration than permabuilding in a hullmod.
Of course, if Alex were to make it so ships only had so many hullmod slots, it'd be a bit more equivalent. And for the record, I don't want a limited slot hullmod system.