Just the knowledge of this would discourage ATM robberies from happening. If you knew that you'd only get x bucks, and that the police would be called as the transaction was happening, you'd be less likely to try this technique.
Well, I think you'd have fewer ATM robberies, but the ones you did have would be more violent, because now the victim is effectively armed with a crude weapon. So the robber is going to be more violent to intimidate the victim into NOT using that duress code.
