About Mages, I'm sadly not agree with you. Indeed, with Cataclysm many spells have changed including Arcane Missiles, "Each your offensive skill has 40% chance to activate Arcane Missile" http://www.wowhead.com/spell=5143/arcane-missiles#changelog (Changelog 4.3.0, because the next is 5.0.1)
That means you have to attack your target with offensive skills (for exemple Fireball ) and if you have chance, it will be actived, else, retry with another fireball ...