I'm sorry, but I have to say that I disagree.
Let's say someone is fighting against Taj's Mewtwo. Every time Taj Dthrows him, he always DI's toward him and gets massive damage because of it. If this person has a coach and that coach tells him "always DI away from him when he grabs you", then that person could win the match just from that. Where as if he did not have a coach, he would've lost.I'm basically saying that coaches can provide information about the opponent's character/playstyle that the player might've not know otherwise. So with a coach, that person can win matches that he would've lost to without one.
Things like this might not be as apparent to the top players, but to lower and mid level players, a coach has a much higher impact on their play and can tell them things during matches that can make them win.
Like Amsah said before, if 2 scrubby players are fighting against each other and I decide to coach one of them, I'm pretty sure that the one that I coach will win (as long as he's willing to listen to me).
Let's say a Peach and a Marth who don't know any advanced techniques are fighting against each other in a tourney match. The Marth loves to Dtilt. If I just whisper in the ear of the Peach player and say "just spam c-stick down", then he'll have a MAJOR advantage and will most likely win the match.
That probably wasn't the best example, but I hope you all know what I'm trying to say.
I just feel that having a coach by your side while you are playing can make you do good things that you wouldn't have done on your own, which means that coaching helps a player to exceed their potential. Using my above example, that Peach player probably wouldn't have Dsmashed him to death if I didn't tell him to do so otherwise.
Yes, I agree that coaching helps push forward the evolution of the metagame, but a tournament match is a whole different thing. I feel that in a tournament match, you are supposed do the best you can with the skills that you have, not by being told what to do by someone better than you (or someone who knows the matchup/opponent's style better). Coaching is supposed to help you prepare you for future tournaments, while tournaments are for testing your own skill.
This is how I feel, anyways.