Purpose for Nonstackable Slows

What was the goal for removing the stacks of slows? Why is it that a skill like Talon's blink that is a 99% slow for .25 seconds override Nunu's Ice Blast that lasts for 3 seconds? All it does from my viewpoint is to make champions with hard CC a higher priority pick more than ever.

Was this change necessary? Why was it needed?