Sorry in advance, but I am hard wired to instinctively 'push back' a little and test any statement presented as an 'absolute' ...
Don't apologize and asking questions is always a good thing,
For a Grav vehicle, the platform is stable and the weapon needs fine adjustments ...
And how do you make those fine adjustments without adjusting the entire vehicle? By allowing the weapon to make adjustments independently of the vehicle.
If one desires a vehicle to fight like a tank...
Isn't that the idea? It's called a grav tank because it fights like a tank. It's not an attack speeder, it's a tank.
I'll also point out that the attack choppers people keep using as counterexamples have weapons in turrets or use weapons which guided after launch. Helos too allow their weapons to be aimed independently of the vehicle as a whole.
Ooops... sorry... that's too much of an absolute statement because unguided rocket pods which are aimed by aiming the helo do exist. Change it to "Helos too allow some of their weapons to be aimed independently of the vehicle as a whole."