I was looking to access a report and was presented with this option. Now, I have read how this facility has increased access to password-protected sites because folk don't have to register on the site but 'sign in' using their Facebook membership. But take a look at what I would have to give up for that convenience:
1 Access my basic information. Well, no biggy you might say, this is all available on a personal Facebook page - errr, yes but mainly only to 'friends'.
2 Access to my profile information. Yep, all the stuff folk can make private from 'non-friends'.
3 Post to Facebook as me. I'll say that again: Post to Facebook as me. Yes, these folk can go to any Facebook page they like and say things and they will be attributed to me.
Needless to say; I didn't 'login with Facebook'.