Thieves can relieve you of access to your entire Apple ID just by watching you type in your passcode while you're enjoying a vodka martini on a night out. Thieves that gain access to your iPhone are then able to pull your access to your iPhone, as the phone doesn't need any kind of confirmation beyond a passcode as to whether an Apple ID is changed from the iPhone. That means they can change your password, and permanently lock you out of your account.
A report from the Wall Street Journal demonstrates a little about what kind of hoops a user then has to jump through in order to regain access to their account, and all the data within.
