Sharapova does not just grunt. She shrieks. Surely the main advantage is not intimidation. It is to disguise the sound of her shot. Her opponent should be entitled to hear the timing of the shot and the kind of sound. Both of these would help her opponent judge the angle and spin of the shot -an early sound suggests an"off" direction and vice versa for a late sound. A loud sound should be treated in the same way as dropping a spare ball - loss of the point.