Email is a generational thing. Older generations didn't have one. A lot of people getting on the internet today don't have one, either, especially in developing nations. But Android needs one, you say -- they'll get their tech friend or someone at the phone store to login once, so they can access the play store. But that's not secure, you say -- they don't necessarily care about that.

The nice thing about phone numbers is you put them into a non app specific address book, so your friends list is portable.