[narwan]

Actually, tanks do shoot tanks with mg's and it is very useful, both in the real world and in the game.

Getting hosed down with mg's forces the crew to button up, which limits their ability to observe the battlefield around them and react to events in it.
While this less of an issue in the most modern mbt's which has advanced c3i, here the advantage of using mg's (in the real world) is to damage these, often vulnerable, systems. Again to reduce the situational awareness of the crew being fired at.

Narwan