Druids (and other classes?) never run short of primary resources

I’m just wondering, a lot of focus in the affixes and properties of items is on regaining primary resource–even though resource is always near full. How can this be explained?

As a leveling sorc I was OOM constantly. My friend plays a druid and with resource on kill and max resource bonus from paragon, still has to stop to get some on attack now and then (switching to resource/sec on gear this patch because of that.)

Not every build uses the same ways to maintain resource, so the variety is useful.

Ring of Starless Sky