Matthew's Journal

Actually, come to think of it, I can think of an almost-sensible use for this which I'd missed. LJ doesn't use only one hostname - some people's journals are at username.livejournal.com. So if (and this "if" is where the "almost" comes in) the whole of LJ were to migrate to HTTPS or to whatever the next fashionable protocol is, then a relative URI of the form you describe would be the most robust way to link to other people's posts :-)