As far as I know, I'm no moderator or anything, but I believe that you UNLOCK new spells with experience points, but you're not allowed taking more than 2 with you to battle. I'll assume you start with 2 spells.
So, I'm thinking, if the new unlocked spells are, well, better, I guess, than others, than the game would become unfair. I think spells should be balanced, like if there are 2 AOE damage spells, one deals more damage than the other, than it should have a longer cooldown.
Or I'll rage-quit, xD.