How can they migrate it to server without requiring login? The best part is that you don't need to create any user for playing. Accessing data between devices requires some form of auth.
Social OAuth and passwordless email login (essentially one-time codes) are both pretty low-effort ways of managing auth without storing a lot of sensitive data or having to manage passwords.
Not for me, absolute deal-breaker. My fingers would find ctrl-w before the "Log in with Facebook!" modal finishes rendering.