Well, i have a friend that have 8 or 9 mp5 ulduar gear. I has 205 MP5. A disc priest without any mp5 have more than this. Any other class will have the double. Why? Because they have talents/skills for that. If they just make Illumination works with mp5 without givving any talent to scale mp5, we are dead. If they give us a talent with mp5, classes balances win again, and fun loses.
And yes, a 6 sec cooldown would completely kill the talent. We would be forced to cast a holy light every 6 sec, because you won't be able to afford losing mana with a crited flash of light.