Passenger aircraft are supposed to NEVER operate with only one person on the flight deck, but rules are not always followed.
We don't know how many hijackings have been prevented by the locking doors. Are there any documented cases? We do know of a few cases where they have caused deaths, or at least prevented any chance to try to avert a disaster.
Could that happen? It's not that hard to know that the doors can be locked, and if so, you pretty much give up on trying to get in to the cockpit to take over the plane.
(Cialdini cites research, which I'm too lazy to look up right now, that claims something as high as 40-50 excess deaths from car and plane crashes in the US per highly publicized suicide)
A co-pilot at the helm, or an unlocked door through which someone may come in and question the pilots actions could very well make all the difference.
Discussing any behavior, especially in the media, makes it more likely.
https://en.wikipedia.org/wiki/Copycat_suicide (Werther effect)
The interesting bit (from the book IIRC) is that the copiers will often wait some time so it seems like they are not copying. This is why the risk to subsequent air travel increases after some delay from the original incident. People are partially unpredictable, so it's possible to have an immediate copy as well.
With seatbelts and their introduction there were anecdotes about people crashing their cars into ponds, lakes etc. where their lives were saved because they were not wearing their seatbelts. I think it is human nature to want to repeat the one incident where the safety measures could have led to disaster.
Since you are so adept at research, can you find this one for me?
Recently a senior New Labour politician who was responsible for bringing in TSA style checks in the UK wise-cracked about how those that complained should be offered two lines at the airport, one with the body scanners, sniffer dogs, robocops etc. and another - to a different flight - where you could just walk straight on. He wondered which flight would be chosen to go on by those complaining types.
It bugs me now that I can't find the quote or remember whether it was Gordon Brown or some other politician.
Now on pilots, this really depends which pilots you are talking about. North American pilots, and here I am pointing at Canadian pilots more because I am in Canada and I am in the process of getting my PPL (for pleasure),however, I am sure this also applies to US pilots, very well trained. To get a job at a big airline you need experience, and where do you get the best experience, far up north where harsh weather, huge winds and tiny landing strips are located. I don't think that 3 years college will do, unless you are lucky.
I also talk to a lot of commercial pilots as my girlfriend is a flight attendant, so during the travels we go out and chat about everything.
As far as safety, over exaggerated, really. No one cares, well, people do but they shouldn't, because if someone wants to hijack I am sure they will find a way to do it. This is the big boys game on a government level. Tax payers are paying for scanners etc, whoever wins the bid makes them and they are sponsoring the next election. (I will not get into this here but you get the point). Scanners are useless.
This is an older video from 2010, scanners give better pictures nowdays but are still as useless as before. I got scanned at Schiphol 3 months ago, the scanner was not much different from the one shown this video. http://www.youtube.com/watch?v=idICUSiGcqo
It is great to fly small planes, no security, quick, and more fun to fly than a big airliner.
Opinions are personal.
"Recently a senior New Labour politician who was responsible for bringing in TSA style checks in the UK wise-cracked about how those that complained should be offered two lines at the airport, one with the body scanners, sniffer dogs, robocops etc. and another - to a different flight - where you could just walk straight on. He wondered which flight would be chosen to go on by those complaining types."
I can't find a report of this ever happening, though I did find an article from December 2001 which pointed out potential danger to pilots behind locked doors. It also notes that the danger of an incident mid-flight can be reduced by having a flight attendant step into the cockpit whenever one of the pilots steps out.
http://articles.sun-sentinel.com/2001-12-17/news/0112160265_...
(Edit: Lest my nitpicky comment be misunderstood, it also talks a little about how strong the reinforcements are; I think your point about cockpit doors being extremely solid can be reasonably taken as fact.)