Can we completely discount the benefit of the deterrent effect on an unstable mind of a second person in the cockpit?
I would say no - it cannot be completely discounted, but nor should it be over-emphasised. It is but one factor to add to the "mix" that a complete risk-assessment requires, and would need expert opinion.
I agree with Ian W that the ramifications of this will be felt and perception will play a part. The press are publishing some more balanced articles, including that "2 on Flt Deck" is not necessarily a good thing. Both the BEA, and now Germany, have announced "reviews" of the main factors.
What I am awaiting is the realisation that the basis for this Co-Pilot being on the aircraft was the regulator, not the airline
Eventually LH will get bored of being blamed, and state who is responsible for, and issued a certificate for, his medical "fitness".