If there is one single addon that can most improve your DPS as a mage, it is the Quartz castbar. Without getting into the technicalities of how it works, Quartz constantly detects your latency (the lag between your PC and the WoW servers). It adds a 'red zone' to the end of your cast bar. What that means is that when your cast reaches that red zone, you can interrupt it and start casting your next cast and the first spell will still go off!

For example, your Fireball takes 3.5 seconds to cast. With Quartz, as you're casting, you see a red zone on the end of your bar which covers ".5" seconds of your bar because of your latency. That means that 3 seconds after you start casting, you can 'interrupt' that cast and start a second fireball. Normally when you interrupt a cast, the first cast doesn't complete, but if you interrupt it close enough to the end, the signal bouncing from your PC to the Server and back doesn't have time to cancel it.

Depending on your latency, you can see a 25% increase in your ability to chain-cast. But let's say that you average only 15% increase. If you saw a talent in your mage talent tree that said "Zero talent points needed to get a 15% increase in DPS (damage per second)", how many of you would take it? Casting faster doesn't increase the damage amount per cast...it increases the overall DPS.
NOTE: after 2.3, using 'stopcasting' macros to interrupt and recast a spell will no longer be necessary. Blizzard is changing the way the spells are handled. Basically, after 2.3, 'button mashing' is the way to go (ie: hitting the spell key over and over really quickly until the spell is finally allowed to fire). Quartz will still be useful, however. With the red zone it provides based on your latency, you have an idea of when to start button-mashing.