Most of what has been said above is true and as a pilot I encourage very open comunication with all the cabin crew.
Since 9/11 we have had a whole raft of regulations about flight deck access forced on the industry by a goverment department that's only aim is to protect it's self from uninformed scrutiny in the press, these regulations mostly uninformed and having little to do with "real" security have the effect of isolating the flight deck from the cabin crew............ exactly what we don't want to maintain good CRM !
The DfT regulators don't infact give a damm about flight safety/CRM as long as there is not a terrorist inccident they are happy, the problem is that a number of the regulations have a detrimental effect safety on a day to day basis yet a terrorist attack is a very unlikely event.
So the only risk assesment that has been done is terrorist attack vs civil servant's career! not terrorist attack vs flight safety.
AS the captain of the aircraft I always take time to meet all the CC and make sure that they feel that they can talk directly to me if a situation requires it, normaly comunication is via the senior CC member but it is important that any one of the CC feels that they can bypass the chain of command if required.
The locked door in CRM terms is problem that can be minimised by good open communication but if this is not done a real "wall" is put up between the CC and the flight deck, this was very apparent in a French airline that we did some work with.