http://forum.mikrotik.com/t/protected-routerboot/85672/1
This feature is not to prevent something from being stolen
This feature is not to prevent something from being stolen. It is to protect your data.
The feature allows to block device from using network boot to access your data without password.
By using protected routerboot, a forgotten password will mean to nullify your NAND, then Netinstall.
This way, if somebody steals your device, your config and passwords are safe.