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While it might be amusing to think about, in the end it just wouldn't be practical. Mages don't have the mitigation needed to tank efficiently. The other classes have tank specs while mages do not. You're a squishy dps/support class with survival cooldowns.
If you wanna try it for fun, then by all means. But I wouldn't get your hopes up if you wanna use it for "serious" content (IE raiding).