It's funny, I just started thinking about this last night, completely independently, while considering one of two Traveller ground combat game designs. This one is based on an adaptation of a space game, of all things.
Yes, a grav vehicle can clearly pivot sideways without changing vector. Yes, there would be more drag, but if you look at the shape of the Trepida, it does remarkably well sideways. Yes, there would be enormous buffeting, but modern fly-by-wire systems allow aircraft that are both statically and dynamically unstable to fly perfectly smoothly. A TL-12+ vehicle computer would handle far worse with ease.
Exactly how a grav vehicle thrusts is a matter of debate, but it's perfectly reasonable to assume two primary sets of grav plates/lifters: one under the vehicle and one to the rear. Additional controls of some sort are necessary to rotate the vehicle; after all, you can't imagine a grav tank that can't rotate in place, can you? These would probably be additional sets of grav plates/lifters, but with much less thrust than the main two. After all, their main purpose is attitude changes and stability control.
Given this, there's a definite reason to have a turret: if you want to accelerate in any direction other than the one you're firing, you must have a turret. Also, if you're side-on to your vector, your drag is going to be higher than if you're facing directly into the airstream. If your rear isn't pointing to allow you to counteract the drag with thrust, you're going to slow down. If all you want to do is take one snapshot, it's no big deal, but if you want to do anything beyond that, not having a turret really is a problem. A turret also allows to your point your heaviest armor at a threat while moving in another direction. Remember, your hull is always going to get some protection from the undulation of the grounds, various ground-level obstacles, and so forth. Finally, consider a really good hit-and-run position. You're placed behind an obstacle with only a sensor cluster peeking above it. Your turret faces the threat (ready to fire), while your hull faces the other direction (ready to run). That brings up yet another benefit of a turret--you can keep firing while you run away!
Actually, MegaTraveller (which is also the first place I recall seeing the Trepida and Astrin drawn out) provides for thrust vectoring off-axis anyway.
Essentially, a "single" thruster (more likely to actually be three in a parallel array) is able to produce full thrust off axis by (IIRC) 30°, and drops smoothly to 25% in a 10° cone. (so (180-(30+10))/75=140/75=1.86, so 1.86°/-1%)... which means the horizontal leg is sin(30°)=0.5 x rated... while the vertical is cos(30)=0.86 x rated. So, minimum thrust is 1.15x weight (not mass - see below) for helicopter like efficiency.
Oh, and MT also notes gravitics can be put into a short duration overthrust mode, allowing up to 400% for a matter of minutes. (under 2 ship combat rounds.)
So, for viffing† or bursts of takeoff speed, they can overthrust...
A dedicated forward thrust plate can likewise be used for additional lift at about 67% thrust... or for Viffing and jinking at same.
The turret allows for bringing the main gun to bear faster than a pure thrust fixed gun can.
The Grav Tank is still, functionally, not a fighter and not a bomber nor fighter-bomber/ground-attack-aircraft. It's tougher than a helicopter gunship (and grav vehicles can be designed to that role), less nimble than a fighter, less payload than a ground assault. It is the merger of the tank and the attack helo, but the role still remains between highly agile standoff and less agile turreted but more heavily armored tanks. Especially since Grav APC's and Grav IFV's are likely to exist.
Notes:
Why Weight? because thrust needs to overcome gravity. Acceleration is mass limited, but lift is weight.
† Vectored Thrust Turn Tightening. Term taken from Car Wars: Aeroduel use.