For me a back is alot lighter then a second camera. I'd love to get an 8x10. The prices for used ones doesn't seem too excessive if I avoid the big name models. I also wouldn't mind a 5x7. The problem is I doubt I'll use either enough to really justify it. A camera with a few backs would let me have one camera but two or three formats. Sort of like using a rollfilm holder.
Wiith a 4x5 back I would be able to take advantage of the longer bellows and bigger lensboard if I need them. Or I could put the 8x10 back on for that special picture.
It just seems like the best thing possible. One camera to carry. A bunch of 4x5 holders. A few 8x10 holders. One set of lens.