For me the worst one I've seen was early in MoP (I think I was around 460ish ilvl at the time) I was in a group to 'zerg' heroic BoT. After the 2nd boss I was pointing out that some people had pretty low dps (mainly a 90 mage roughly the same gear as me pulling 6k dps, I was pulling 50-60k). They proceeded to flip out on me saying that meters are stupid and I should not be concerned with their dps, to which my retort was their low dps meant that we wouldn't be able to ignore boss mechanics to kill fights. The group didn't make it much farther, we got heroic council down but could not kill cho'gall because we lacked both the coordination to do the fight proper and the dps to zerg it.