I'm still stuck on "how well should an instructor play?"
I think it's theoretically possible that a wheelchair bound quadriplegic could intricately study all aspects of the game and become an instructor worth listening to. But at the same time, I can't quite put my finger on why I remain skeptical that any instructor that's below say -- 600 FR could help me, or any advanced player, enough to justify paying for their services. Guess I'll try to put my finger on it...
I know that Fargo Rating list has been around for a while, and some of those ratings aren't fair but there are enough instructors with low enough ratings to entertain the question.
There seems to be only 2 reasons any of them could give for having a lower rating: 1. At some point in their pool journey they gave up on becoming an EVEN decent regional player and began coaching instead or... 2. They've continued playing and their instructor tool kit is limited to the point where it doesn't help their own game.
If 1 is true, I think there would always be at least some level of disconnect between what they are TEACHING as correct and what they KNOW as correct. If you haven't been to the mountaintop, describing it properly will always be lacking in detail. If you don't have a tried and true method that you have used to increase your own skill to a certain level, you will HAVE TO have some level of faith in someone else's technique that you pass on to your students. This seems less than ideal to me.
If 2 is true -- well that's just alarming.
A regressing rating is totally different to me and no cause for concern. That's just life.
So in my book the instructors that have played at a high level and communicate well -- I'd put at the top. Guys like Demetrius, Stan Shuffet, Beeler (just based on what I've seen here in the past few days), maybe someone like Allison Fischer, and Alex Lely, who I spent some time with a few years back.
Below them, you need to bring something to the table that compensates for not having reached the mountaintop, while still exhibiting that you can play respectably. This is where there's room for superior knowledge. Dr. Dave and Bob Jewett fit this bill, and I'm sure there are a lot of others. They both play well and I suspect above the 600 rating litmus that I've proposed (at least for myself).
Why 600? Maybe you could convince me to go down a little lower, but when you get around 600, that's when your pool eyes fully open up. Before that, it's hard to really distinguish good from great, and if you can't see the difference, how could you lead a player to greatness? There are things in life, you just can't comprehend unless you've experienced them. There are things I think I see when watching SVB play. Like little idiosyncrasies that I think could actually be flaws, but I guarantee if he was to explain how these things feel to him, it would be different and I would probably be the one learning something.
That's just how I see it.