Common sense says that it is a bad idea to have a single person locked behind a door that can only be opened from the inside. It costs nothing to have a cabin crew member go into the flight deck when one of the flight crew needs to leave and it provides a level of protection should the remaining pilot suffer from some sought of collapse. I find it hard to believe that it is not already common policy by all thinking airlines. It has nothing to do with trusting pilots and would not stop a determined deranged pilot from pointing a plane at the ground but it might just make the deluded person think twice when they are not alone.