We too have the CCTV but it doesn't cover this: Pilot goes for a pee. Other pilot becomes incapacitated. Door entry system fails and we have an operational pilot in the cabin with the pax and unable to get back to the flight deck. End of aeroplane and everyone in it. Sure, it's a long-shot - but not impossible. That's why my Company always has two people on the flight deck in flight. It's usually a cabin crew member but if there's a third pilot, that fulfils the requirement. It's a simple, easy, safe procedure and costs nothing.