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.
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.
1: http://avherald.com/h?article=46c3abde&opt=0