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It takes too long to level up! But why?
+20% to magic damage? A Battle Mage gives the same bonus! And then you can level up other skills, like ranged attacks, or something else!
No, it's not about that at all. You're not getting the point (as they say nowadays) :) These +20% are really insignificant. The whole point is that you have several effective spells for each situation. Each spell lacks something. For example, Haven lacks protective spells or blessings of Life, and Nature and Chaos lack spells that remove enchantments, etc.