I guess I would ask why? In theory, it would be safe / secure / anonymous. You have to remember fewer passwords, and this site would never ever see your authentication. It also wouldn't know your email address, only that you have been authenticated by a trusted third party for your specific account.
You as a user get to choose which 3rd parties you authenticate against as well, so you can avoid google or facebook if you don't trust them, and go with yahoo, or someone else who supports it.
Is your reaction based on superstition or is there some merit as to why you would dislike having that kind of integration?