Well yes ack ack. They would be easy to take out. Then again, M3s shouldn't stop HE rounds, or produce ricochets regardless of the angle and area. Ack guns should be killed by a few .30 hits (representing crew kills). And most tanks should die in one succesful penertation of the armor. The bulkhead would keep fire from engine damage out of the crew compartment for a while, but eventually ammo will start to cook off.
The game is full of unrealistic crap, and stuff that should be deemed a waste of resources (WW1 arena, IMO), and yet we still have it. Its how you pick and choose, remember.
And my bet is that people would find tactics that work with them, just as most kills using the M4A3(75) (that I've seen) are made or started by damaging the turret, and then damaging the engine/killing the tank when it turns to run. After that, they seem to scoot away as fast as they can, which is a smart decision given their firepower.