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.
Everyone likes to say that classes are too similar. They're not. I don't care if every melee has an interrupt, or if x-amount of classes can bring a heroism -- the classes themselves play almost completely differently. They're not similar.
This is one of those things a random Blizzard person said once, offhand, and has since been taken to be the gospel truth behind their entire philosophy. The reality is that Blizzard don't balance around "bring the player not the class", especially not when it comes to heroic raiding. They expect a whole bunch of different things from a raid comp. Just in ToT for instance, you were meant to have good cleave/multidot on Council, a warlock for slowing turtles on Tortos, a whole bunch of raid CDs for Megaera, strong AoE that is good on walls for Durumu, basically ele shamans and warlocks, lots of multidot for Animus as well as a third tank and healers that do well while the raid is spread, lots of melee for Iron Qon, multidot for Twin Consorts and two death knights for Lei Shen. Oh, and minimal melee for Ra-den.
Some things like that are accidents in the process of translating a fight to heroic level tuning but most of them are expected. They know guilds can bring 8-10 multidot for a fight if they want to so they tune expecting at least, say, 6. If they didn't, you could cheese the fight by bringing a larger amount, which is why 25 man heroic guilds need flexible rosters.
A better way to think about it would be "have as wide a variety of classes and specs as you can to cover every eventuality, and once you've covered all the basics the fight needs, bring the player, not the class". What they want to avoid is a situation where you bring all the stuff you need and fill the rest of the raid with warlocks, it's not just "bring any 25 people you want and win".