Hybrids, however, do, and that means that paladins, druids, priests, shamans, and to a certain extent, warriors and death knights, will in Blizzard's view never be able to equal "pure" classes in terms of DPS output, with everything else being equal. You may love your ret pally, and he may be in uber gear, but he should never be able to pour out as much damage as an equally specced and geared hunter, because you can switch to healing, and the hunter can't.
Is that wrong? Not at all -- it may disappoint some people (and Ghostcrawler probably knows, but we'll remind him anyway that posting even this clearly on the forums won't put a lot of complaints to rest), and it may cause some rerolls, but it definitely makes sense. Flexibility is an asset, and not having that flexibility in a pure class does mean that they should get out of paying that tax. If you only do one thing, you should be able to do it better than someone who has the option to do all three. And as he says, what you want to do with the class has nothing to do with how Blizzard is balancing them overall. It doesn't matter if all you wanted to do when you rolled that priest was play shadow -- they need to be balanced as hybrids.
GC says that Blizzard has played around with roles in the past, but going forward, this is the philosophy: if you're playing a hybrid class, you'll never be able to fulfill a role as well as if you were playing the "pure" class meant to serve that role.