Ever seen sites with the ability to connect via Facebook? It often grants said site(s) with the user's Facebook primary-email. Now all personal emails, including password recoveries, are going through Facebook for said site(s).
I'm having a hard time imagining a scenario where a site would send some information via email but that same information would not be available to anyone logging in via the web interface. But whatever.
You seem to lack imagination when it comes to nefarious deeds, which is fine; unfortunately, facebook does not lack imagination in this area (and in fact, one could argue this is a core belief at facebook, since it was founded upon a hacking incident wherein Zuckerberg borrowed student data).
They desperately want your email...they don't want it because it's cool to be an email provider. They want it because they intend to use it. The point isn't what specific piece of data they'll get from it (though passwords will be among that data--as a mail server administrator of 15+ years I can assure you of that); the point is that it's simply evil for them to interject their servers into the path via deceptive means.
