I was a bit playing with SUL functionality and tried to register my account on projects which I never registered before. I could login instantly, but I tried to create an account the old-fashioned way. I did use different passwords etc.
The greatest advantages are single sign-up (...) consistent identity (your username now always means you; no one else can take your name on another project).
As far I can see for now, this isn't the case. I am still able to create accounts (though I didn't try this with other usernames) I don't know whether this is done intentionally (I couldn't find any statement about this), some IP-check is performed, or this is really a bug.