For me it's one of the most important aspects in a rapper's arsenal, if not the most important. I simply can't listen to someone who can't use their delivery to express the emotions the track itself portrays in the right way.
I'll take Andre 3k for example. His charisma and presence make him really enjoyable to listen to, even if the beat that he's spitting to is simple and provides close to zero musical experience and enjoyment.
This was just a thought I had earlier today so I figured I'd post here. It would be nice to read your opinions. Peace.