An unregistered account doesn't have any associated accounts, which means unregistered accounts doesn't increment the number of committed users with a reputation higher than 200 in other sites. There is also the possibility that, before the proposal passes from the commitment phase to the beta phase, the users lose the account cookies on their account, with the probable effect that they will create another unregistered account to commit again to the proposal.
What is the benefit of allowing unregistered account to commit to a proposal? It doesn't seem to be done to catch more experts. The experts Stack Exchange is looking for are experts that should use a website to answer to questions; I would guess that for such experts creating an account on Area 51 is not a problem. If the expert doesn't know anything about Internet, or website, that is not an expert that can help a Stack Exchange site.