As long as it isn't your only authentication method I don't see harm in adding it.

I'm extremely turned off by sites that want me to sign-on, with the only option being Facebook, because I've had enough of sites demanding irrelevant permissions they obviously don't need (you need my email, not to be able to post to my wall). If I really want to sign up for something I'll do that and 'fix' the permissions in Facebook but it really peeves me.