As a side note it will now leave hundreds of cabins unattended every day when the pilot of a 50 seat aircraft uses the lav. (1-50 rule)
This requirement is only for evacuation on ground and does not affect cabin operations while the aircraft is in flight.
Having a cabin crew or extra crewman sitting on the seat while an operating pilot is out of the flight deck only precludes the possibility of someone 'unwanted',wanting to gain access or will lessen the risk of an operating crew-member being locked out of the flight compartment as such was the case in the GermanWings accident.