As I could tell, there were no specialized roles for anything except artillery. Big tanks are faster, better armoured and pack more punch than small ones. There is no point in using any other type of tank. Tank destroyers doesnt take as long to level up, but are severely limited with their low armour and lack of turret.
The whole MMO thing just looks like an attempt to hide a substandard game that lacks the gameplay variety to keep people interested, if they didnt have the "I bet the game will be fun once I unlock a big tank!" carrot to sucker you with. If it was just a multiplayer game like Battlefield or Red Orchestra, no one would play it.