You're wrong. First of all it's not a "grunt" but a full out scream. Second, a simple forced exhalation of air while striking the ball works better. Screaming actually takes more energy then a true exhalation.
They scream because it masks the the sound of the ball hitting the racquet (which is actually very important to judge the ball spin) and because they think it's intimidating.
The screeching should be banned.
Guess you’re not a big fan of vuvulezas either? ; )
This is a VERY true statement. I listened to a lecture by John Newcombe and he did a wonderful demonstration about the different sounds made by top spin vs. back spin. He is one of many that feel the scream is nothing but a masking technique to hide the shot. He agreed it should be banned.