The ultra-light guys get by basically with the stuff we pack in our survival kits and generally they do just fine. The difference though is comfort and safety. I don't feel comfortable that I'm prepared for anything without certain gear/tools. Like someone mentioned before, you may only need your first aid kit once, but once can be the difference between making it to get help and not making it out at all.

Here are some ultra-light gear lists:

http://onestep4me.tripod.com/gear.htm

http://www.stanford.edu/~rmahony/ultralight/gearlist.html

http://www.ultralightbackpacker.com/weekend.html

http://www.gossamergear.com/gossamergear/images/gear_lists/Gear_List_Summer_Sierra.pdf