Mages are intended to be solid in PvE. In fact, most of their issues have come from the fact that they are asked to hold still too much in PvE in order to turn out good numbers. I don't really think that "Noxxic's rankings" is a great thing to judge on. Raidbots overall isn't so bad. "Raidbots top 100" tends to show mages overperforming, mostly because fire is an RNG spec (aka, the same mage's performance can vary more per pull than if he was playing, say, a mutilate rogue with the same skill). I would dismiss arguments made at mage PvE performance with these valid reasons, instead of something that doesn't have much application outside of tabletop gaming- or at the very least, outside of group PvP.
The "glass cannon" idea is built from a different mold. Do you really think that in arena, the mage is normally the kill target, for instance? Do your mages normally die on whatever PvE fights are progression for your guild? Does it take more skill to survive as a PvE mage than a PvE warlock, or death knight, or fury warrior?