with low light performance of the 5n, what is the appeal of full-frame?

I don't think you need to be a pro at all. I suspect there are more amateurs who can benefit from FF than pros (in raw numbers) and it's not even close. My preference is to shoot with the best looking bokeh or out of focus areas I can get on the vast majority of my shots. Gaining lesser DoF on FF is a real benefit for the subjects I'd like to shoot. I can see it very clearly in the photos of some of the bokeh related groups I am in on Flickr when checking out the cameras involved in the shots I like most. The difference is significant and that's without being a pro or shooting sports or political events.