Two angles here.
Firstly, German Wings example where depression/mental illness is the causal factor in crashing. In this case, having another person on the flight deck is prudent and our policy works. Very few people take their own lives with others around. An act often done in private although there are exceptions such as those that throw themselves in front of trains or off bridges onto cars. The second example being alluded to is the terrorist. As already stated, nobody, even a qualified pilot, would be in a position to intervene if the timing was correct. Even a four man ULR crew could do little.
This is probably why authorities are taking differing views, given the differing threats faced by individual airlines. As a high profile brand and prolific sponsor of Western sports, EK is hardly the model Muslim airline. Combine our location in the highly volatile ME region with our propensity to employer a large percentage of Arabic males, and it's clear that the policy we have is probably not the best.
Harry