Since I like to try out a ton of different services, I normally end up with a bunch of accounts and a bunch of passwords. I don't exactly like this. I'd much rather be able to try out the app and then decide later if I want to sign up. (This is one great advantage of OpenID, but you still have to leave information on the service)
This does two things in my opinion. It would save me from having to worry about information leaking (email address, etc) to services that I'm actually not interested in using. Two, it potentially saves the product developers from getting their hopes up when they see tons of users signing up at launch, only to realize that most of them never come back.
I really like the way jaanix.com takes care of this, or rather used to take care of this. They gave you an account like guestXXXX or something strange, that could then be converted into your real account, when you actually sign up.