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.
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.
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.