Sounds cool, but only as long as purl.ly itself is up. We’ve seen what happens with single point of failure services like this when twitter’s link shortener t.co was down.
Actually, if you notice, the purl.ly link has the original link after it. Even if purl.ly goes down, there's at least SOME reference to be tracked down. Relatively graceful, especially when compared to a WebCite link: http://www.webcitation.org/5IfzstWm1