I leveled mine fire/arcane. Fire first, so that single-target DPS was still great. Arcane later when I had enough points to get both Pyroblast/Blastwave and Improved Arcane Explosion. Problem with speccing straight for arcane up front, as I see it, is that you lack a lot of the single-target DPS or control. If you're gonna quest level, that won't do you much good. If you're just gonna AoE grind level, then spec whatever makes you happy as it's really hard to spec wrong for AoE as a mage.