The point in this instance is about fun, not realism.
However if you want to go into this realism thing right now then heres how it is Mr Tenacity. All projectile weapons require ammunition, like it or not thats the truth.
Rounds don't just appear in endless amounts inside a real life tank, nor do torpedos just magically spawn themselves inside submarines. Therefore you arguement based on realism about specialisation and escort (blah blah blah) for NOT having ammo for GB missiles is frankly absurd.
In my opinion however the far more important thing here is about the fun we have playing the game. Now I can say from experience that it is absolutely no fun whatsoever being randomly blasted by 12 missiles in under 5 seconds. Missiles that track rather well, missiles that are quite powerful. Not fun at all... Especialy in a vanilla fighter.
I can only assume that using no skill whatsoever to beat a comparatively defenceless fighter is fun for the pilot of the GB, otherwise I would assume they'd stop using full missile turrets... One or two missile turrets on a GB is fine, but full missile loadouts is really unbalanced.