For example, KDE and Gnome automatically lock the screen in 2 minutes unless keyboard is pressed or mouse movement during the locking prossess. If the locking prossess is only stopped by hiting some combination key, I think no one can guess the combination key in short time to stop screen locking. Thinking about boss from SilverRoad, if he had the screen locker like this, the police have no idea, he might not be sent to the jail. And certainly I'm a normal user, no need to lock USB Stick.