Now unless they open the door just how are the cabin crew meant to bring in the food and drinks, and / or how are the flight crew meant to be able to use the toilets and or stretch their legs ( assuming here that we're not immune to DVT ) ? ........ and of course as soon as that door's opened we're vulnerable !
Bottom line is that the locked door policy is a waste of time, becuase all any self-respecting terrorists have to do is to wait for the right moment for it to be opened during the normal course of flight operations.