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):
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