Fullwings: I stated in a prior post (there have been several on this issue) that the two-crew procedure for US carriers was there to mitigate a lack of both video cameras and magnetic locks on the cockpit door. I agree with you that an FA posted in the cockpit might not be a fully effective deterrent for a rogue single pilot. As others and history suggested, in some cases, neither has a fellow pilot in the next seat.
I feel the procedure is now being implemented by EU and UK airlines for two reasons (aside from new regulatory guidance) 1. no airline wants to be considered less safe than another (ticket sales) and 2. the airlines need to show some form of threat mitigation was in place in order to limit liability in case of a similar incident occurring down the road.