You're right that they are probably just edge cases. As a thought experiment I see three possible use scenarios - 1) Lose the phone and it dies 2) Malicious person gets phone, tries to restart for whatever reason, and is locked out 3) I know for some reason that someone is about to try to get data off my phone in person and I have time to turn it off.

More to the point there really isn't a downside. Entering a password when I restart is nothing for me, and the read/write slow down isn't going be noticeable because I don't ask much of my phone.