Problem is it made ultimate generation too powerful for some builds, resulting in constant ultimate spam. The primary culprit was bat swarm vampire dks, but I remember being able to literally lay down two veil of blades with my nightblade in dungeons back when this was a thing.
The current system ensures roughly equal ultimate generation regardless of class or role, and also allows them to more easily balance ultimates because it's more or less consistent how much ult you'll get in a given amount of time.