How many mages do you know that went pure frost spec from the start? There are a few Arcane talents that are pretty sweet for those levels, and any mage that doesn't get Improved Arcane Explosion ASAP is insane. Just about every mage I've ever played with went arcane/fire until 60. The only exception I can think of to that is if a mage is AOE built which would mean arcane/frost(at least if they are smart) which I did until 50, and then I went to arcane/fire until 60. Now I'm arcane/frost again.