Have you seen the threads where people complain they can’t get invited to a 15 because their spec saw no play in MDI? When every spec in the game is perfectly viable at +20. If we got rated duels, you’d see a lot more high rated healers, rogues, and arms warriors than a lot of other specs, and there would be complaints on a mage tower level.
It’s a lot easier to fit 1 specific class out of 12 into a 20 player group than a 1, 2 or 3 player group.