It changes the sport. There is no question you could help a player with some coaching. But as it is now, our sport greatly favors a player who can think for himself and keep self motivated for every point and doesn't need somebody else to tell him to hustle, not give up, change tactics, etc.
In tennis, players try to break each other over the course of a match. And by break, I mean tactically, mentally and/or emotionally. It changes the game if a coach is there to protect/shield a player from these breakdowns.
I like it how it is. I like that tennis a game that you must find your own way, start to finish.
You can play a million practice matches and coach/stop play all you want. But official matches are truly one-on-one and it's something that separates tennis from every other sport and I'd be sad if it changed. (and in fact, I think its sad and embarassing and actually demeaning that the WTA has oncourt coaching while the men eschew it.)