You'd be in various situations where someone nearby you whether a friend or a family member would want to use your laptop and there's nothing you can say but say "yes", even though it isn't a problem. All the apps are closed and running in the background and there aren't any notifications popping up unlike the iPhone with 5.0 so your new incoming information isn't exactly showing up to their eyes.. However with Mountain Lion they bring the Notification Center to the Mac and add this for the convenience of the user. What would be the possible options of creating some kind of privacy for the main user? Apart from limiting popups I can't imagine any. Obviously you've got a guest temporary user that one could use but is there anything else?