Captain H: if that's your worry, then maybe you should both take separate flights...
White Knight: I am not so worried about crew (cabin or flight deck) jump seating, but would rather not have relatives, and definitely not random passengers who are victims of an overbooked flight. As an FO, I have FA say in the matter, despite being asked with the old "Are you happy with that?" to which I reply "No" and am always overuled, due to the fantastic CRM ethic in the company that I work in.
The flight deck is my working space, it isn't very big, and I do feel self conscious with anybody sat on my shoulder. I would prefer that they weren't there. At least another crew person will have the deceny to keep quiet whilst I am working. This is quite a different reason than a security issue, I grant you.
Also, the policy has been decreed and decided in my company, that no one other than positioning crew are allowed on the jump seat, and that the door must be locked. Unless Flight Ops Director says it's OK. Which seems to allow anybody not covered by the above to travel on the jump seat. If the FOD or Captain says it's happening, then that's final. As I say, as an FO iget FA say in the matter, other than the formality of being asked and ignored.