The problem with this is the legality. See, technically, if it's on your computer, you're responsible for it. If you've got hundreds of thousands of dollars worth of warez and the feds come to your house, it doesn't matter whether or not you knew it was in there. I mean, you did download the client, didn't you?

sucks, but there's little way around this. A data cache is a great idea, but it would be made better use of if only certain systems (supernodes for example) contained one. Not only are the bandwidth benefits obvious (some people don't want to share thousands of files), but those who run such supernodes have already accepted the responsibility that comes with helping to purpetuate file sharing.