I think we all know that stopping cockpit access has done nothing to really enhance safety onboard the aircraft. I don't think the the history of jet aircraft that a welcomed guest of the cockpit has ever tryed to takeover the aircraft EXCEPT for the Fed Ex DC-10 incident, however in this case he was a company pilot, in uniform, someone who is still in most airlines still allowed access.
Having a guest of the crew, a close friend or family IMHO provides no decrease in the safety, infact the only thing it may do is provide another line of defence against an attack on the cockpit, in a/c such as the 737 someone sitting in the j/s impedes immediate access to the operating crew.