Arguments against mid-set coaching
I'm going to start by making some rebuttals of common arguments in favor of mid-set coaching.
1) "Coaches promote long-term player growth."
I agree, coaches do promote long-term player growth. However, mid-set coaching does not promote long-term player growth in any way that *between set* coaching (ie Tafo in Melee) fails to address. The long-term skills which coaches help develop - match analysis, a positive mentality, setting and following goals - are all things which happen between sets. In fact, there is a strong argument to be made that mid-set coaching HURTS long-term player growth. Why? Because it discourages players from actually expanding their own game knowledge - why learn more about the game when you can just ask your coach if you're ever faced with something unfamiliar?
If someone claims that they want mid set coaching to be legal because it will help them grow as a player, they're lying. They want mid-set coaching because it helps them gain an advantage over their opponent and win the set (if they believe they have access to a better coach). The kind of coaching which *actually* helps promote growth as a player has always been legal, and always will be, because you can't regulate what happens outside of tournament sets.
2) "Real sports have coaches, so we have to have coaches too if we want to be taken seriously!"
This is an absurd argument which stems from failing to understand what coaches do in real sports, and failing to understand the difference between individual and team sports.
A team sport consists of many different people filling many different roles. The coach is simply another one of these roles. The reason this is used and necessary for team sports is because players on the team do not have omniscience over the rest of their team. A player on a soccer team cannot be expected to know exactly what the gameplan of all 10 other players on their team are, and cannot be expected to know what everyone else has been practicing. The role of the coach is to integrate the knowledge of all the different players on the team so that they can function cohesively. Mid-match coaching is essential to team sports, because otherwise you don't really have a team at all, you just have x different independent units trying to awkwardly fit stuff together.
When you look at actual INDIVIDUAL sports:
-Tennis: Most men's tournaments prohibit mid-match coaching. Women's tournaments are a bit more blurry.
-Chess: "Players may not use any notes, outside sources of information (including computers), or advice from other people. Analysis on another board is not permitted."
-Fencing: Varies between tournaments. International events seem to allow coaching between bouts, while others do not.
-Badminton: No explicit coaching rule one way or the other. Seems to vary between tournaments.
Golf & boxing are the only individual sports which seem to unanimously allow coaches. Other than that, for most individual sports which are remotely comparable to Smash (i.e, a direct competition between two people, not a massive group contest like triathlon) it seems to be either outright prohibited (Chess & Tennis) or up to the discretion of the TOs (fencing, badminton, most other things). There is absolutely no precedent that "all real sports must have coaches." Stop citing it.
Now, onto the crux of this post:
I think the biggest argument against mid-set coaching is that it can cause players to be punished for what is traditionally skillful play. Suppose you have a scenario where a Pikachu is playing against a Fox.
-Game 1, the Pikachu player uses cross-up landing fairs to pressure shield.
-The Pika notices that Fox always tries to shieldgrab, not expecting the crossup.
-So, the Pika - being a good player - makes note of this. He starts utilting after his fair crossup to catch the Fox when his shield goes down.
The Pikachu now has a read on a specific habit of the Fox player. This is a skill that should be rewarded. So suppose the Pikachu wins game 1, and the Fox calls his coach over. The coach tells him - secretly - "hey, he's always doing crossup fair, stop trying to shieldgrab it." The Fox player takes note of this advice, but keeps it hidden until the Pika is at 80%. This time, the Pika does crossup fair, and the Fox punishes instead by doing an utilt -> uair for a kill. The Pikachu was actively PUNISHED for the fact that he had a successful read on his opponent. This is not something that is conducive to helping players grow. Players should be rewarded for winning the mental battle between them and their opponent; *not* for trying to predict what the other guy's coach is going to whisper between games.
Of course, there's also the fact that mid-set coaching is, by its very nature, anti-grassroots. If I'm a young, unknown, broke college student with tons of natural talent, but I'm playing against some sponsored player who's hired a really good coach, I'm automatically at a disadvantage. Allowing mid-set coaching encourages stagnation at the top, as it will always be easier for the popular, more well-established players to have access to the better coaches. Of course, this is a criticism that also applies to between-set coaching; however, regulating between-set coaching is impossible, since that occurs outside of tournaments. But prohibiting mid-set coaching, at the very least, prevents this problem from being exacerbated.
I know there are many knowledge & lab monsters who want to be relevant in the community by being a coach for someone who actually gets results in tournament. To those people, I say: dedicate your time into something productive, like streaming, commentary, or TOing, or start an actual between-sets coaching service where you give lessons, teach people to improve their mentality, and analyze sets. Trying to force the community to embrace mid-set coaching simply that you can feel notable / help out your homies when it:
-provides nothing objectively good for the game which isn't already provided by between-set coaching;
-punishes players for being smart; and
-furthers the class divide between "established players" and "new players"
is not the way to go.