No matter how ready to plug-in arrangement they can provide for your mobile phone, digital camera etc, they can still be a cause of potential hazard. One can get an electric shock, if accidentally gets in contact with the bare end.
Take this incident, a Toddler Trinity Anderson was idly playing with a plugged-in but unused USB cable when she (apparently) put the bare end in her mouth. She got an electric shock and promptly got unconscious.
Although adults are unlikely to do what Trinity did and even if they choose to do so, the effect will be much less (output of a USB cable is only 5 volts); but the shock hazard is still there, more so in case of improper or insufficient grounding. --------