* It starts instant and saves time. Normaly I have a mob down with arcane missiles bevor the first fireball would have reached him.
But it also means you pay your mana up-front. If the npc dies on the first tick of 4x missiles you wasted 80% of the mana.

* A casting mob who recieves an arcane missile every 250 milliseconds has more trouble to canalize his own cast.
NPCs don't lose cast time from damage

* Since I'm specified on arcane explosion, arcane missiles profit from my arcane tree talents.
You have water elementals. That means you have 41 points in frost. That means you can't get any of the arcane talents that help arcane missiles (Arcane Instability, Arcane Potency, Spell Power, Empowered AM, or Mind Mastery)


Still, watching 4x AM spam definitely looks cool!