While I understand what you are getting it, if you have teammates getting hit by plume it's better to let them die, as they are likely to make the same mistake many times later. I've not once run into a situation outside of my static where I thought "Gee, I really need .16 seconds off of my cast time to save that dumb DPS player who keeps getting hit."
I can see Spell Speed being a build when we get into the 700/800s, for when you get that high you'll have close to a whole .50 off of your GCD, and that is huge!