Not sure the situation in other countries, but in the US you aren't liable for any money lost in an ATM robbery. So this would be helpful to the banks, but not the victim.
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.
