You don't get IAE on the way to Evocation. While it's true that frost is not as efficient at low levels, it's awesome once you get to 40 and go full frost. My frost build maximizes on shatter; you get great mana efficiency, and when you pair up with another frost mage, it's insane. I do have Evocation, but I always drop it between 40 and 51 for frostbite, etc.