While I agree that it's unbelievable that this thread has gone on for so long, an experience at BHX recently also served to show the futility of the exercise.
As I was boarding an AFR flight to Lyon the gentleman behind presented his boarding card to the stewardess with a loud and cheerful, "Off to Amsterdam today!" Puzzled silence ensued with a query as to whether he was flying BHX-LYS-AMS or BHX-AMS. As I had to wait for the people in front to get bags and coats away I heard the whole exchange, with the gentleman saying it was funny, normally his company told him when it wasn't a direct routing, and the stewardess agreeing it was indeed strange, but he was probably going to change in Lyon, so welcome on board.
He then proceded to the seat behind me, and proptly got into an argument with the person boarding behind him as he was sitting in the other chap's seat. He then made it back up to the front of the aircraft and last I saw of him was him scurrying across the apron with a ground handler. How he ever thought this was the right plane I don't know, because the gates only get put up at the last minute at BHX, so it's not like there are a list of other flights going later from the same gate to get confused over.
Without wanting to reignite the security debate again, I was never convinced of the "it's for security" argument, and after that little episode, am even less convinced of it. Actually, I can't really think of any good reason for it, especially if you can board with the wrong destination on the boarding pass anyway. But as it's no skin off my nose to hold it in my hand for a few extra minutes after going through the gate. I am one of those people who does show it and I do smile - frankly it takes more energy to be unpleasant.