Tanks might throw off gameplay balance but would add so much to the mood. Think of playing on destroyed village, crouching behind some rubble with your trusty rifle. The creaking sound of a tank slowly rolling through the streets is near. It stops... the cannon slowly swivels towards you. You've been spotted! Then it's time to run! The rocket launcher would also be able to take out the tanks.
And think about tank battles with the infantry running between and throughout the fighting tanks shooting at each other. That would have such a kick ass atmosphere... but you're always gonna have those people that will whine cause they get shot or run over by the tank alot.
Couldn't be any worse than getting fragged by rockets too many times.