Maybe I'm old school but I still see tennis largely as an individual sport with a big mental component. To me it is all about problem solving and a contest of both mind and body.
I wouldn't want players in the middle of a chess or scrabble game to be coached as it would seem like cheating and the same should go for tennis.
And as the parent of a 9yo who has just started playing tournaments and competitive matches a few months ago I feel that I have ample chance after a match for myself or his coach to do a proper debrief. So when he walks on to a court in the next hour, day or week he is just that little bit wiser and has had a chance to digest what he needs to learn.