Why did the Colts rest their starters?

That whole "resting the starters" is such a myth, yet so many coaches in the NFL seem to get caught up in it. Why ruin a team's mojo? Who's to say you rest Peyton for the game, but gets his ACL tore the next practice day? Since you can't predict that stuff, there's no point in trying to prevent it- if it's going to happen, it'll happen.