[NiHaoMike]

Motors generally don't mind square waves - even the Prius drives the motors with square waves at high speed, where road noise completely drowns out any motor noise that would be produced.

The longer answer has to do with the inductive time constant of the motor. The inductance will tend to round off the current to be more like a sine wave. If that's not enough, the sharp edges make vibration and noise. High end 3D printers are now starting to use sine wave drive, since they often operate at low enough frequencies that no motor would be inductive enough to smooth it out.