I am genuinely surprised by the number of posters who have raised the possibility that CC can be so readily infiltrated by terrorists that being alone in the cockpit for a short time with a CC member could present a credible risk of terrorist action ... yet having that same person with an airside pass and free to move about the aircraft in flight is OK.
Because we do know how easy it is to get that airside pass, sadly enough. Usually CC are cheap rented labor from some third rate agency, there for a couple months and then gone again. Yes, there are some airlines that still do employ most of their CC on real contracts and do not rent them in on an ad hoc basis, but i believe that will probably vanish as the other way is so much cheaper, especially if there is no business/premium product on offer.
The reason I made the contrast was that the CC in the cockpit answer is the standard in the US and doesn't appear to have suffered from any of the issues raised in this thread.
In my opinion there is a cultural difference. As far as i know the CC is required on both sides of the atlantic if there is no video system to identify the person in front of the door and make sure that the area is clear. Those systems are the norm in europe (apart from ryanair) but apparently weren't in the US. Therefore our US brethren are used to the CC in the flightdeck, we were used to not needing them and working without them, since they are not required (and trained) to open the door from the inside. Even now that they are supposedly trained most are not able to operate the electronic lock, not to mention the video system.
The whole thing is a farce to be honest, the result is that pilots do drink a lot less during a work day and dehydrate to prevent interrupting the passenger service and get some disgruntled CC and another one blocking the galley. If they have to do it anyway the remaining pilot has to cope with that disgruntled untrained person on the flight deck and still operate the door/video system anyway. It is simply a measure to placate the travelling public, not a real security measure.