Arcane Explosion is a relatively unique spell in WoW. It's about as close to zero sum as you get. If Arcane explosion is enough to kill, it's worth it, if it's not, it's basically useless. By this I mean, if you stack enough mages to where you can frag a mob out, you can frag out any number of similar HP level mobs at once, and you can do it quickly. Which means, that often times arcane explosion is situational at best for a lone or even duo of mages, but get 3, 4, or as the people who were raiding instances before the nerf, 5-8 mages, and suddenly you are able to kill 20-30 or even entire instances in a single pull. It's not terribly mana efficient, and this is why it's usually not the most effective way to spend mana for a lone mage. However, if you got enough to kill with it, then you can kill everything with it, at once.

Other AOE's, tend to be slower and more efficient, or simply not viable like Holy Nova. Physical aoe is also typically hit capped, to just a few mobs. Which is why stacking mages is basically broken as a leveling mechanic. They are playing by different rules than everyone else