Yeah, an underquilt seems cool aside from the expense. But I prefer having a pad because what if there's no trees? I guess if you're hiking on the east coast it's not likely to be a problem but almost all my backpacking experience has been in the middle east where maybe there's trees, maybe there's not, and it's drilled into my head to look for the campsites when they come, and to be prepared for when I can't find trees in the right configuration.
What's a "footbox?" And how would a shelf in the hammock work?
Anyway, like I said, I'm not looking to buy another hammock, I'm just wondering what makes people want to "upgrade." I still kinda have a hunch it's that same bug that gets everybody who gets into any pastime, like how one bike is ok, but once you get that second bike, suddenly you need like fifteen.