This Thing You Call Aggro.

[LochnarITB]

The reason Arcane missiles is good is that it meters damage out over time.  Scorch allows for relatively mana efficient DPS, which is why they are used in instance situations.  These allow the tank an opportunity to build hate while you do damage.  A fireball straight off will frontload all the damage, while AM meters the damage out over several seconds.
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I don't think it is what you meant, but I just wanted to add that this doesn't mean to let loose with Arcane Missiles immediately. Because they begin to fly right away, if the tank has not grabbed aggro at all on your target, even the small damage of that first burst will pull the focus to you. The cast time of other spells creates a built in delay before the damage hits. With my frost mage, I can usually start throwing snowballs when the mob is down 5-10%. I also do not fear pulling aggro once the mob is close to death. If the mob is ready to go down, I can finish them with a set of arcane missiles. Sometimes, tanks and healers panic when a squishie draws a mob even at that point. If they are aware that the mob is close to going down, trust the mage to finish it. Don't chase it down or go into emergency heal mode.

Also, since the early levels of Lochnar, I have thought of my little squishy as being expendable. If I see a fight going south, I will pop a shield (love my Ice Barrier) and go in to do as much damage and drawing as much aggro as possible before going down. I will, hopefully, have pulled aggro off the healer and taken a few of the baddies close to death or dead. Mage aggro can be a good thing, especially if you pair it with the cry "LLEEEERRRRROOOOOYYYY JJJEEENNNNKKKIINNNSSSS!".