Tactics: Why pull players with fouls?

[Stargazer]

Both coaches faced the same problem in the pivotal final minutes of Game 4: Bench Amare/Duncan with five fouls, or let them play? Both pulled their stars, and both teams suffered for it. Arguably, Amare's return before Duncan's cost the Spurs the game.

But why do coaches always do this? After all, both players finished with only five fouls, meaning that in hindsight, at least, we know that both players could have played more minutes without fouling out. By pulling them, the coaches only guarantee the very result they are trying to avoid.

[Obstructed_View]

Since Duncan came back into the game and completely failed to play any defense or even try to get a rebound or two, I'm not sure why Pop would have bothered sitting him in the first place. At least Amare was aggressive on the offensive end when he was in.