That makes more sense now but I still don't see why having a square wave will damage the power transistors?
If you were saying that, there will be a point when both transistors are on at the same time, allowing a large current to flow, then the circuit isn't designed correctly: there should be a dead period when both transistors are conducting anyway.
Anyway, I think you should read up on modified sine waves to find out for yourself why they're used rather than square waves as you're clearly not going to listen to me.