Maybe this has been covered elsewhere before, but what exactly is the articulated rationale for leaving the curtain open or not having one there at all? How is this supposed to enhance security?

It seems to me an open curtain is just an invitation for coach passengers to see what is going on next to the cockpit door and when, to stroll through first without anyone thinking much is out of the ordinary, to go up to the F lavatory, and potentially take one more step into the cockpit.