The benefit for me is that you get to choose who stores your password. A random website may store my password plaintext, I have no way of knowing -- so I am forced to use different passwords for each site, and manage them some way.
Ofcourse, I _know_ this, but in general, users don't know and don't care. So, OpenID actually makes the web a safer place for inexperienced users, which is I think is rather important.