Whether they're good, bad or indifferent post-Cata I don't think matters much.
I'd suggest that if you're leveling a mage solely to use it as a drinks and portal vendor, then it's probably not worth the effort.
I think Blizzard will be making sure that each class will be viable post-Cata just as they are pre-Cata, so I wouldn't be worried about it, and "the sky is falling" threads on the WoW forums can be safely ignored this far out.
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