Please correct me if I'm wrong (I don't actually know how the 'Find my Mac' feature works, so I'm guessing), but when the computer is first booted by a thief, it won't be online, which means that it won't be aware that it needs to disable access to all accounts except to the neutered guest account right? In fact, the only way that the computer will be made aware that the mac has been stolen is if the thief connects to the internet (Apple has done nothing so far to enhance the likelihood of the user connecting to the internet). Once the Mac is connected and is aware that it is stolen it's behaviour will change to block all access to user accounts. If anything, this change in behaviour might alert a thief that something is up (I'm giving a lot of credit to theives here) and that he should rid himself of the Mac.