Facebook forces all users over to Facebook.com e-mail addresses 14 years ago
Well, that email issue on the phones seems to be a design issue with the OS, and not Facebook's overall fault.
On my Windows phone, if I have a Windows Live contact (the base contact saving system) with an email, and then port in Facebook contacts that match, it merges the accounts into one "profile" and it doesn't overwrite emails; it saves both emails and tells me where each one came from. Maybe if Android had a better system, this wouldn't have happened?
It's not Facebook's job to take something like that into account.