But speaking seriously, everything is relative, meaning the best magic will be that which you consider most effective in a given situation.
Indeed, using "Fireball" more often makes a decisive difference in a battle with an opponent, and thus in the game as a whole. A sacrifice can turn the tide of battle. In utopias, for example, it is more rational to use "Summon Elementals" and not lose units. I have never read or seen absolutely anything about the "Magic Mirror". I cannot imagine how it is used in real battles?! And, by the way, 7 computers on a team at 200% is not an argument — for many, I think this is not a problem!