As a tennis player myself, I'm a grunter, though not a very loud one. It's more like a sigh.
Yeah, I think it should be legal. Though with Sharapova (hot as she may be

), it's annoying as hell. Her grunt doesn't even sound "natural", more like she's yelling intentionally to distract her opponent, which is lame.
As a grunter I can say that it comes from the effort put into my swing, not a yell.
Within the game of tennis though, there are three types of grunters:
1. Those that grunt on every shot, be it serve, return, or whatever. Every time they take a crack at a ball, there is a grunt.
2. The ones that grunt on specific shots. Roger Federer and Pete Sampras were like this, in that they both sometimes grunted on their serve, but not on much else.
3. The completely silent ones. Roger Federer nowadays, as opposed to his younger days, is more like this.
I personally think the best Tennis grunt belongs to the Brazilian player Gustavo Kuerten. Watch this video:
Link
His grunt is just so funny.
