[Zelphyr]

Increasingly there are going to be people like me who don't trust Facebook, Google, Twitter, etc... enough to have an account (or, at least, a real one) with them. So using them for logging in somewhere else isn't helpful.

ONLY being able to use them to log in somewhere else is obviously a reason to never sign up with that "somewhere else" site altogether.

[Nursie]

And that's not even taking into account that the random dragging in of resources from these places allows them to track which of their users visit which other sites.