Serious problem.
There is a growing awareness that aircraft are vulnerable when one pilot leaves the cockpit.
If you allow any form of access from the cabin, for instance to avoid a Helios situation, then it might be used by desperate cabin crew during a hijacking. If you don't then either pilots can not leave the flight deck during the flight of if they do the aircraft is vulnerable.
There haven't been nearly as many hijackings since armoured doors were installed but I am a little surprised that once they were mandated no one took the time to think it all the way through.