First post - and undoubtedly not as helpful as I imagine, but as a passenger this incident has not reduced my trust in the skills of professional pilots, and their ability to get me safely to my destination - this despite the certain knowledge that with the right circumstances, something similar will happen again.
Which in a way is the whole point, because with due respect to posters who have made a range of thought provoking contributions to the question of the second person on the flight deck, perhaps the pertinent issue here is that of circumstance.
There’s very little credible doubt that Lubitz deliberately crashed the aircraft, but despite lots of conjecture, no clear motive as to why. Without that motive, the root cause of his actions, it isn’t possible to target future preventative actions aimed at weeding out the potentially dangerous entrants to the profession, or in identifying them and mitigating the threat once they are already qualified and flying. What you can do however is change the circumstances in-flight which may contribute to not just a decision to deliberately crash a plane, but also help facilitate it happening.
It is not true that all acts of extreme behavior are conducted in private, but it is true that most perpetrators of such acts have waited for the right circumstances to fit their needs and intents. In this instance, nothing could have served Lubitz’s needs and intents better than an empty flight deck, and his actions as we know them appear to demonstrate that he needed or wanted to be isolated and alone to do what he did.
Would he have seen a member of the cabin crew present as a deterrent? We can’t know for sure, but we can know that it would not have allowed him to act in isolation, nor with certainty that he was going to succeed, when that crew member could have opened the door at any time for the aircraft commander to re-enter the flight deck. A crew member on the flight deck with him would have changed the circumstance, and thus dynamic of the situation, to something certain to be less constructive to his intent, and that may have been enough to save 149 innocent lives on this occasion, either by deterring Lubitz from his planned (or unplanned) actions or by letting the commander back on the flight deck in time.
What seems certain is that if another Lubitz in the same frame of mind were to be left alone on the flight deck again, the result would be more loss of life. If another crew member present were to save even only one in ten of those flights, it would be hard to argue against the policy.
Of course, that raises the question of how competent cabin crew members may be to identify and resolve such difficult circumstances, and I can only speak as a passenger with far more experience of cabin crew than of pilots, but I find them remarkably capable of handling problematic and stressful situations and difficult people with speed and efficiency. For sure they wouldn’t know how to operate controls on the flight deck, and it wouldn’t be reasonable to expect that capability, but from having to deal with the traveling public in such numbers and confined spaces, they seem very capable of sizing up people when they need to. I would trust (most, at least) to grasp what is happening on the flight deck in the kind of situation we have seen happen here.