The problem is that Facebook's claiming that email is just another piece of metadata about a user. However, from Google's point of (and rest of the non-Facebook web), email is not a metadata, but the key identifier around which all of the services know which "John Smith" the given user is.

If I'm building anything interesting and want to play outside the Facebook walled garden, the Facebook ID doesn't really get me anything - it's like someone giving me a currency from a random country that I never intend to visit.

Unfortunately, even Facebook itself treats the email address as an identifier data (what do you need to use to log in to Facebook? Email address and password). It's a little disingenuous for them to turn that around and say that it's a metadata that's part of the user's profile, as opposed to the only unique, persistent, and widely used identifier that we currently have on the web.