Well, I wanted to separate crew and gun to make rebuilding howitzers less expensive. You pay a high amount of MP and it gets deleted by one click
I think ferwiner means the gun should just come with the crew to keep it simple and the crew should work just like USF's, but you of course believe the crew should be the only units capable of crewing the guns. I'd say to do both - make the crew come with the gun, make the crews the only ones capable of using the guns, make the crews able to build an unmanned gun (and only them) so their survival is paramount to recovering costs from destroyed guns.
You could make them better by nerfing the 3 units that completely negate crew-weapons.
Pwerfer
Walkingstuka
Calliope
Off-map arty is a significant factor to their current lack of usefulness as well.