Everyone so far seems to have given the expected response

I wouldn't use it either but there are benefits to it.

You don't have to trust sites to store and secure your password. In the event they do not properly secure your passwords a hack or database leak could mean your password is public and can be used to login to other sites where you have used the same password. But you should be using a different one for each site anyway
Using login via Facebook etc means that is not a problem. There is no password to leak.

A site could also set it up in a way where they do not record your name or personal info from the details retrieved from your social media profile. Just your user Id. Though you would be trusting a site not to do this.

A site could aslo setup the login system on a non gambling or bitcoin related domain so that Facebook etc don't know you are logging in to a gamvling site. Sure they could figure out eventually though.

A player could also create fake Facebook account to use for password but then if they are going to go to the hassle of doing that then they probably are the type to use fake email and unique passwords etc.

I was just curious as I was writing an OAuth login system at the moment and wondered if this is a feature to add or not.
As simple as it is to do so it probably is a bad idea and could even put some people off using a site.