I figure if I reserve it for use in an emergency, when the time comes it'll probably have expired, gone flat, or I'll have forgotten how to use it, or whatever. You need to practice and test. Using it for every day stuff at need is one way to do that. Plus if you are putting in the work of carrying it you might as well get some benefit from it.

What elements of a PSK are, in fact, useful daily is a separate issue. I'd normally have water so I wouldn't use water purification, for example, but my flashlight, compass, knife etc get used all the time.