As A Moderator from the Brazilian Forums, we've opened the EXACTLY same pool there.
Our main argument to get Coaching prohibited during matches is what DMG just stated above. Telling the player you are coaching to stop rolling too much, stop spotdodging that much is fairly OK but still changes a bit the match since the opponent might be abusing/punishing from those flaws the player you are coaching are making, which, obviously, he should realize and correct them by HIMSELF ONLY. Still make this kind of coaching bearable.
But, lets suppose some coaches are telling the players he is coaching, about specific and unique patterns every player have on his playstyle and how to overcome them or telling him "Be careful Wario's Waft is up to 1 minute!" or "C4 just under right platform" could change DRASTICALLY the match since the coach is putting in danger the opponents playstyle making change/adapt it only because someone has told to do this or that. Thats really annoying, unfair and over all probably, stupid because you are playing against X not against X+Y right? 1x1, not 2x1, if the coach didn't talk about those patterns the opponent could probably win the set just by keeping up his solid playstyle.
Sorry if this writing got a bit confusing. I'll be checking this frequently to be sure you all guys (which want to) get it right.