There are lots of scenarios that can "separate" you from your backpack. Being pinned under a boulder is one of them

http://www.youtube.com/watch?v=HYrqs3wh3QQ

Although this guy may not have survived with a PSK alone and was lucky to have met the other backpacker , but the sudden unexpected mishap that happened to him shows us how ANYTHING can happen while outdoors and you find yourself unable to access your bag even if it was a few yards away from you.

It is very possible that a guy like this one goes to have a leak at midnight, gets attacked by a bear or something or breaks an ankle and gets injured or disoriented in the dark , and has to wait till morning to see his way back to camp. Not a bad idea to wear a fanny pack type of PSK when moving around at night.