I answered this (as best it can be answered without delving into SimC's source code) just the other day Mage Simple Questions/Simple Answers [Cataclysm]. Forum rules require you to at least check the last few pages of the relevant threads instead of being lazy and just posting your question.
According to the generic ilevel 372 profiles from simulationcraft.org, an arcane mage's best secondary stat (once hit capped) is haste. I have been looking for a reason why this is true outside of simulation, but I cannot find one. My simple reasoning suggests that haste will simply make burn phases shorter and conservation phases longer. I would have guessed that crit is better due to the mana return from the tier 1 fire talent as well as being more efficient through doing more damage. Why is haste good for arcane?