I can safely say coaching in-between points has little or no effect on most players most of the time. At the most, coaches shout words of encouragement to the player(s), a reminder,and only occasionally give cursory tactical advice.
As for tactical advice/instruction, if it is to affirm what was told to the player(s) earlier at the game break or mid-game interval, fine, nothing changes. If the coach suddenly mouths something different, a mere word or two without details, what is the player(s) to make of it? Worse still, it may serve to confuse the player(s) and be counterproductive.
Usually, in the heat of the battle, nothing gets into the head(s) of the player(s). If the player(s) is/are leading or gaining points or at least not losing points, they just carry on the same strategy/tactic. If losing points or trailing badly, nothing the coach does can have much effect on the player(s). During play, the player(s) are practically left to their own devices, to think for themselves and make adjustments accordingly.
The only time when coaching can make any difference and achieve the intended effect to a certain extent is between games and during mid-game intervals where the coach can go into details and after an assessment of the match proceedings. Don't be surprised, many players half the time aren't really listening, it's in one ear and out the other.
LYB once said that BCL was too reliant on his coaches during matches. At the LOG'12 Lin Dan vs LCW showdown, LYB said he sat at the coaches' chair purely to lend LD moral support and, hopefully, put some pressure on his opponent by his presence. I supposed he is implying that a good,hardworking student with the right attitude and has done his homework well, doesn't really need much coaching during matches, except for the coach to give pointers, reminders or encouragement, and perhaps when facing an unfamiliar opponent or one who suddenly plays a different kind of game; that's when the coach watching at the courtside has the advantage of a spectator's view of things and,therefore, may see more clearly than the player involved, the better to give him useful advice.
But, at the junior level or for young,inexperienced players still at their formative or (early) development stage, needless to say, good,quality coaching is very necessary and important at all times and in all situations. It then depends on how much the junior players can absorb,understand and implement the advice/instructions/guidance.
In conclusion, I'm basically neutral as far as coaches giving advice in between points is concerned. If I have to vote, it's a "No" if only to avoid confusing the player or, worse still, make the player develop a crutch mentality.