I feel that my online ID is andrewducker.livejournal.com - it's been the centre of my online social life since 2001, it's where I keep in contact with my distributed friends, where I post things I think about. In a very real way, it's my defining online identity.
And now, thanks to OpenID, I can carry that identity with me to other sites. If I post on Stack Overflow I'm posting as _me_ - not just as whatever user id I happened to get in the gold rush.
That matters to me. If it doesn't matter to you then that's fine. But it's important to me.