Generally speaking, the more information your opponents have, the better off they are, and so in a zero sum game, the worse off you are. By that logic, showing your hand actually hurts you since it gives your opponents more information.
Since a typical game of poker lasts through many shuffles of the deck, however, the information they have gained is probably only useful for a relatively short time. What I think is the more important factor here is the psychological aspect of the game. By showing the other players you have bluffed, you are messing with their minds.
There are only so many probabilities involved in poker, and people can basically learn them. In my opinion the real skill in poker is knowing how to play the mental game.