I don’t think they are a waste of time or useless at all. Like any other exercise they need to be used in the right way. As Paul states above, I use them and hip hinging for people who have knee problems and it just hurts to do other exercises. Great for muscular endurance. I’d be sure to hold just above or just below parallel to the floor if there are not knee issues.
I also incorporate one legged wall holds, then the client has to focus on more core stability too. 2 legged wall sits can be made more challenging by adding lateral band pulls on thighs and pressure from my hands.