What are the advantages of being an unregistered user?
The main advantage of being an unregistered user is that you don't have to sign up. So if you feel sketchy about OpenID, or simply have only one question that needs attending to, it's a great way to get expert answers without tying yourself down to some place.
Sometimes, people don't have the time to dedicate to full participation in a Q&A site, and by allowing people to act unregistered we are able to provide our services even to these people. We give them an easier avenue than, say, registering at a random forum. It's not a big hassle, but not having to do it at all is very advantageous.
Ideally, these users do the smallest part of payback by accepting a good answer, but we don't even enforce that on them.
Aside from the lack of ties, the only other "advantages" are not having to deal with the downsides of being a user, such as: