First of all, I'm not saying it's bad to bring the best player. But I don't think it's very interesting or even realistic to be able to bring just any class.
I understand the reasoning, it sucks needing a mage or a warlock or whatever. But that made each class feel more important and unique.
IMO what they should have done is not gone to the extreme of being able to bring any class, but rather need a certain category of class. For example, where you would need a mage normally, you could bring either a Mage, Warlock, or DPS priest. Instead of needing a rogue, you could bring any melee DPS. And instead of being able to have 3-4 pally healers, you need at least a pally/monk AND a priest/shaman/druid.
I think all this has done is made classes too similar for really not much of a benefit. Like I said, it isn't interesting at all and makes each class less important when they're replaceable with whatever.