Some lessons can only be learned, not taught.
Others can only be taught by those who have been there themselves.
Yet others can be learned by reading a book or watching a game.
Would I want a coach/instructor who bangs balls into the rails, and has done so all his life? Nah, don't think so. I mean, if the person had obvious brilliant insights, I'd give them a chance, but I don't think I've ever met someone like that. When you hear that prolific pool authors can't run even a single rack, it gets you thinking...
Every "theoretical" rule of the game I've ever seen gets broken by almost every pro at times. This game is not about those things. It's about playing to your strengths and letting your talent and subconscious do the work. This goes for fundamentals as well as theory of other kinds. There are guidelines and concepts that are valuable to learn, but should not be worshipped, but stored in the back of ones mind. Patterns for straight pool is an example. Once learned a pattern can become a part of your natural game, but you should not force your game to conform to patterns, especially ones that people who can't run 3 balls recommend. Likewise other "hobbyhorses" that are frequently touted as the only truths should be tested and discarded if they go against your natural tendencies.
If your game is garbage anyway, you may as well try, but don't ruin your game to conform to a theoretical standard that no pro or decent player follows.
I've discovered things about this game that nobody teaches as far as I know. Some don't teach them because they themselves can't play well enough to know them, others don't because they can't describe them well enough or do not want to. I find these truths to be ones most worth having. I can't teach you the "perfect" grip, because our hands are different. I can't teach the "perfect" stance, because our bodies are different and some things cannot be conveyed in words or even with pictures. I bet the pros know many more, that we mere mortals will probably never know and couldn't even use if we did know them.