It's a passenger confidence building measure that seems not unreasonable to implement from a
PR point of view at the present time. Probably worthless in reality, even with guidance given to the cabin crew, how do they know if the remaining pilot is about to do something dodgy. It does seem a little silly having to call the L1 when you need to go peepee!
The reality is you can probably never stop a determined mad man or woman from doing anything, particularly if they are a psychopath without empathy or remorse. A jump seating pilot would definitely have a better clue of what to do.