My mage always specialized in conjuration. My reasoning was that so much of what she does has to do with summoning stuff (including pets), that it made sense to go with the specialization that saved her some mana in the long run. It also just seemed to be in character for a mage.
Several of the better nukes at higher levels are conj based instead of evoc. That's probably why people who soloed a lot with evocation tend to switch when those spells become a bigger part of their ********
If I had to guess, though, I'd say the recently nerfed pet cannibalization was a big part of the switch. Mages could recover significantly more mana than the pets cost with improved canni, and if the mage specialized in conjuration the margin widened a bit. That doesn't apply so much any more since a recent patch, however.
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