I only dabbled with OpenID as an end-user, but I really really liked the fact that I could tie my identity to a domain I owned, and then temporarily delegate the credential-mechanics to another entity.
In this way, the identity was durable and portable, at least as long as one maintains control over the URL.
Emergency reserve military duty, a sudden sickness, email delivery issues, or even a temporary financial problem at the wrong time of year could all be reasons to missing the domain renewal window. It's happened to people far smarter than me.
Sure there's the spec, but it's a lot to keep track of for the intern that inevitably ends up implementing such things.
Having a test suite you to verify you didn't mess up would be very useful.