This is so true. We had to shed some weight (4 players) due to them not vibing with the guild plus they would refuse to alter their methods for bosses. 3 were dps and all had meh dps, like I was beating a fire mage! This week we did the skip to sire without them and man what a difference in how smooth the fights went.
You dont have to be the best dps in a raid but you do have to pull your own weight and at least attempt to show/care on improving.