Damned if I can see the "obvious reasons". The reality is, the UK has a positively anal attitude to any people on the flight deck, even pilots. I believe the US also follows suit. But vast chunks of the rest of the world have a more enlighted attitude. Yup, the wrong person is a obviously a danger, but I feel safer with my wife, kids, neighbours on the jumpseat than a new recruit who has been assigned to be there.
So to and from airfields in the UK FIR, no pax on the jumpseat.