With regards to the argument of 'why do you need a gun if you're behind an armoured door?', what people fail to realise is that the cockpit door is no undefeatable barrier. Access can be gained by obtaining a key (in todays society with todays technology not unfathomable for a determined terrorist cell) or perhaps by breaching the bulkhead in the forward lavatory?
In my humble opinion a suitably trained, armed flight deck memeber is an asset to security. Just because of one incident which we dont even know the outcome of the investigation yet, there are no grounds to slate it. We are all pilots, qualified to the stadards our respective lands have set out and we have all made mistakes and had incidents in aviation (big or small) but yet we see them as lessons to our selves. Lets do the same here. As pilots it is our duty undertake the flight in the safest way possible, unfortunately in this day and age maybe this has to extend beyond our airmanship to other avenues.
Unfortunately we live in a world where there are armed threats against us. You cannot counter an armed threat if you are unarmed. The cockpit door will not always save you. In the US as in South Africa where I am from we understand this. Perhaps in the EU and UK where armed threats agaist you are not as a common occurence/historical occurence it is harder to understand the need to carry a firearm.
I for one would love to live in a gun free state, thats why many of my countrymen have immegrated, but I choose to live here so as long as the criminals are armed, my .45 will be by my side. If our CAA allowed armed flight deck and if my country and airliner was under constant terrorist threat as in the US, it'd be there too.