When I was underage in tech school we used to do this but one of the guys was a bad alcoholic and I had to cut him loose and move on. He just kept saying "why?" smh, he couldn't see it. It wasn't a waste though. If you want to improve you have to take playing opportunities when they come to you and a rehearsal is a playing opportunity. Rehearsals and gigs are never convenient but how bad do you want it?
These days I don't play in bands I have my own group so I get to be the boss. I don't do sub work either. I won't even play shit I don't want at a jam I'll just put my guitar away. I even got called a primadonna once, lol.. I figure it's all so much headache I might as well play what I want to play and heck with everything else.