I understand what OAuth does...more or less. I would predict that my sister does not. "Google" and "Facebook" don't connote privacy. They imply that if a person types "shoes", the next four hours of browsing will be full of shoe ads. That's the dissonance.

Why have a login at all?

Login is clearly aligned with privacy when "privacy" is mostly defined in terms of authorization. Login is less aligned with privacy when it is defined in terms of anonymity.

Google+ and Facebook are both services built on removing anonymity.