Bringing a Gun to a Knife Fight

Who didn't enjoy the movie scene where Indiana Jones blasted a sword-wielding opponent with his six-shooter? If he's got a knife then just shoot him - right? Isn't that all there is to it? It always works (and looks) great in the movies, but what about in real life? What about the real-world officers both patrol and correctional as well as other operating professionals who really have been there and tried that and it didn't work? Why is the "21-foot Rule" such a critical factor and what can you do to beat the odds stacked against you?