While I have no doubt that the MPL pilot in the RH seat of a big jet will have superb skills at monitoring the automatics and press the right CDU buttons with great aplomb. But what will always concern me as a passenger down the back is the dark and stormy night event when suddenly the auto pilot clicks off and the radar screen is full of big red stuff. Will our MPL go heads down and dive into furious frantic button pressing, when the priority is to hand fly the aircraft to get in back to an even keel with real black night instrument flying - especially when the flight directors have gone crazy and become meaningless. It may never happen of course but put yourself as an experienced pilot sitting nervously down the back taking wife and kids on a holiday and up front things are looking decidely grim with one captain and one inexperienced but proven highly competent button pusher. But of course it would never happen, would it....
Imagine being the captain of an ocean liner or an aircraft carrier. Can you ever imagine the Second in Command of these ships being in that appointment with the equivalent of a bare MPL.. It would never happen.