As I said before the only purpose of UserGuid field is to relate to mp_Users ie site members, it is only supposed to be used for unsubscribe for site members and it is also used to pull the site members subscriptions into the user profile page.
For non site members the Guid field in mp_LetterSubscribe is supposed to be used for unsubscribe (not the UserGuid field which is expected to have empty guid). However there is a bug in the current release where the wrong field is used for the unsubscribe link for non site members who are subscribed to plain text email.