But why not just use OAuth/Facebook Connect to both sign up and log in to the service (where username collision makes no difference)? Why require a username and password in addition?
It's really in case Facebook "screws them" -- in that scenario, nobody is tied to the Facebook login and can still access the site. Allowing them to Facebook Connect now lets them at least take advantage of viral "things" -- e.g. a wall post is posted when you sign up.
