Scott- Off the top of my head, easier to change tubes for flats, switching tires, easier storage in cramped cars or airline luggage, and as Tirving suggested swapping different wheel sizes in the same frame.
Edit there are a couple of older threads on QRs
http://unicyclist.com/forums/showthread.php?t=37414&highlight=quick+release
& Koxx-One prototype
http://unicyclist.com/forums/showthread.php?t=88345&highlight=quick+release&page=3