"Never" show? That's just silly guys. I'm not saying you should show just to make yourself feel good (ex showing a strong hand that got cracked so you can show everyone how unlucky you were), or anything like that (same w the quads, btw). But there are situations where you want to show.
You should have a good reason to show, and that reason should usually be premeditated: you might make a play intending to show your cards at the end no matter what. The classic example is reraising a maniac or lag with total trash, showing the hand no matter what happens (he folds or you fold to his rereraise), in order to slow him down regarding attacking you.
You can also use the show to set up an opponent for a future play. Show him a pattern, change it up on him when all the money is on the line.
TJ Cloutier makes a point of showing all his strong hands early in a tournament, making his 'I only play monsters' image very pronounced and thus exploitable later when it really matters.
There are pros on the other end of the spectrum, like Eric Seidel, who never show. And that is good advice as well.
So I think, like most things in poker, saying "never" is a bit myopic.
Don't show without a good reason (usually preplanned), and don't show if you are inexperienced, or are not sure exactly what you intend to accomplish by showing, or are not conscious of exactly what information you are giving out, or if the information you reveal will be more detrimental than helpful to you in the long run.
But don't say that no one should ever show any cards ever....