Ghostcrawler has a nice exchange on the forums (and it's not marked "Not Tracked", so you know he wants us reading this one) about how much of a role class representation actually plays in class balance. Obviously, balance itself plays the biggest role in class balance -- if (using GC's completely hypothetical example) Feral Druids are overpowered as tanks, then Blizzard would have to look into nerfing Feral Druids.
But what if Feral Druids only make up a small percentage of the tanks in Ulduar? Should Blizzard nerf the 5% of tanks that are Druids down until they're only 2% of the population? Just because classes are overpowered doesn't mean that actual class populations are, and all of these things go into the mix when Blizzard makes decisions about how to balance the classes.
Look at Hunters as well -- just last week we talked about how the class population is falling off, and yet they're one of the top three classes played on both factions. So should Blizzard buff or nerf them?