So for mages specifically, they have been pretty clear at least in some expansions that you're the CLASS before you're the spec. You're not "FIRE", you're a "FIRE MAGE." So you specialize in Fire, but you still have your basic mage abilities. Same with Ret Paladin. You do damage, but you're a Paladin before you're Ret, so you can still heal and provide utility in blessings.
I agree though that it would be kinda cool to see Mage be more like the WC3 style, but I think we're too far down the path for something like that.