Michigan coach Mark Rosen talks here about creating depth, and he makes a point of saying he doesn't buy into the one-mistake-and-you're-out philosophy. His view is that building confidence in your players, whether they start or come off the bench, involves letting them work through some mistakes so they're not "looking over their shoulders" and worrying about being pulled from the match.
Of course, the trick is finding a balance. If there's a player on the bench who can help, it's necessary to get him or her into the match before it's too late.