While I agree that most people don't want one person/team winning all the time, I believe that this does generate more interest, ie. Many people will take a vested interest in a baseball game so that they can cheer against the Yankees.
Cheering against the Yankees would be a waste of time if the Yankees literally won all the time. It would increase people's interest in the game the same way that a trick coin increases people's interest in betting on coin flips.