One tank even left in the start saying "I only do fast runs" when we politely said that we need to kill all bosses to gear up.
He did smart thing, no reason to paint him as bad guy. His time wasn't wasted on something he doesn't enjoy(slow run), your time wasn't wasted on something you don't want(skipping bosses). Can't really blame him for that, as it's better than demanding that group submits to his will. He leaves and gives you chance for someone that will be willing to do same thing as you want.
I can understand the need to skip regular mobs, they dont give anything useful in general at level 50
Not true. They allow you to play the game. Kill stuff, tank stuff, heal stuff. They give experience of doing that stuff. Just like people might need gear, they also might need practice. And running around mobs doing everything in your power to avoid aggro range can get just as boring as fighing 57th trash pack