[narwan]

Artur, I'm sorry I'll have to disagree with you. The OP fire draining is not a 'disease' of the game engine. As c_of_red already pointed out, OP fire draining simulates an age old tactic.
You're also wrong on assuming an APC can't harm a tank, at least your reasoning is faulty. APC's carry infantry. Infantry carry weapons that can kill tanks. So tanks kill APC's whenever and whereever they can. Cause they don't know if the APC in question actually carries infantry or not. The tanker has to assume it does.
Same for light armor, it may not threaten the front of your MBT but if it can work around your line, most 'light' units carry enough punch to penetrate the side or rear armor. You're also assuming the tank (crew) think only of themselves. They will probably figure that that light unit will be a threat to some of their colleagues and try to take it out since they often have the best chance of doing so (and in relative safety).