From what I know, all my Embermage spells are all instant cast, so how or why would I want to stack +cast speed %?
Spells are by no means "instant cast" for the Embermage. Most every spell has some sort of cast delay, one of the most prominent being (my personal favorite) Death's Bounty. The second tier for this skill has the following perk (emphasis is mine):
5 soul bolts are released and casting speed is increased by 50%
Reaching this tier is vital for use of this skill, and the improvement in casting speed is obvious. Not reaching tier 2 may result in your not being able to cast it fast enough to save yourself when battling higher-level mobs. Casting this spell on a mob is essential to keeping your health and mana high enough to continue casting attack spells.
So, long story short, stacking
+cast speed % will cut down on the "warm up" times for all spells, allowing you to do that much more damage (or improve your defense) per second. Note that you can still play the game and have fun without using
+cast speed %. I never stock up on this kind of thing, and I get by alright (though I'm not playing on Elite, nor am I playing hardcore). Having fun is what's most important, and with this game, that's hard not to do.