The Zen of caching by Lucas:

A half decent cache will still work when you're offline, the content goes away, or you don't feel like downloading! One difference is that the download and rename becomes invisible to the user. But the other, really crucial, difference is you can keep referring to a cached song by its URL, which allows you to share pointers to it with other people. If you cache rather than download, the song is still part of the web.

I like that - If you cache rather than download, the song is still part of the web. I had never thought of it so clearly, until Lucas said it so simply.