NigelOnDraft: Understood, but are you really suggesting that an airliner pilot must refer to a cabin crew to approve every decision?
I ask again - to those who want a second crew member in the Flight Deck what is their role?
To open the cockpit door?
On 9-11 I was flying as a captain for a major U.S. carrier. After going through some interim procedures to prevent breech of the cockpit door (serving carts blocking the aisle with flight attendants standing behind them, F/C pax volunteering to tackle anyone trying to get over the cart, etc.) we finally got the reinforced doors. We then went to having an F/A stand just inside the door anytime one of the pilots needed to use the lav. Their sole functions were (and are) to raise the alarm in case the sole pilot at the controls collapses, and to physically open the door for the returning pilot in case the electric lock mechanisms malfunction.