Believe it or not, people do forward text messages. Both of the biggest mobile operating systems have a feature that specifically allows you to do so. I doubt this would be the case if the feature was not popular amongst common users.
I understand this point of view but feel that it assumes the ‘average Facebook user’ to be thinking from a tech perspective. This is obviously not the case. As creators, we should know better than to trust users to do what we expect.
The link gives you access to anything you would be able to access if you were authenticated via conventional login. You’re right, you still have to re-enter your password if you want to change sensitive information. However, you can still do everything else: send messages, upload pictures, write on walls etc. We cannot look past the potential consequences of unauthorised Facebook access. It could ruin lives for all we know.