P-51s have the advantage of looking innocuous to anyone but TSA thugs. You can carry one to places where you'd be shot for having a SAK. Plus, the rubber band on it is really useful.
this is my main reason for carrying one. these days you never know when or where you'll be asked to leave your knife behind. with a p-38/p51 on the keyring, you've at least got SOMETHING to use as a tool. i cant' find the page, but i believe someone here posted a very MacGuyver-esque list of things you could do with one.