Interesting Denti. As a general rule in the US it has been a mandatory for over a decade.
As i understand it the main concern is identification and door operation. Those are available by other means. All airlines i have worked for have video surveillance of the door area and forward galley as well as something akin to the much quoted CDLS. As backup, if either the video surveillance or the electric door didn't work the two crew rule had to be used, as otherwise the pilot would have to leave its seat to open the door or identify whoever wants to enter. That is the reason apparently why ryanair used the two crew thing, they simply didn't pay for the video surveillance.