Can you think of any war prior to Iraq and Afghanistan where the United States announced to the world when it planned to pull its troops out? What has this accomplished? "By their fruits ye shall know them." What have been the fruits?
First of all, this constant talk in Washington about not only pulling out, but announcing in advance what their pullout timetable was, meant that Iraqi political leaders knew that a powerful Iran was on their border permanently, while Washington was a long way away and intended to stay away.
Should we be surprised that the Iraqi government has increasingly come to pay more attention to what Iran wants than to what Washington wants? Once more, vast numbers of American lives have been sacrificed winning victories on the battlefield that the politicians in Washington then frittered away and turned into defeat politically.
What about other countries around the world who are watching what the American government is doing? Many have to decide whether they want to cooperate with the United States, and risk the wrath of our enemies, or cooperate with our enemies and risk nothing.
There is no need to respond to a North Korean artillery barrage by wiping North Korea off the map. But there is also no need to reassure the North Koreans in advance that we won't.
What announcing the doctrine of "proportional" response does is lower the price of aggression. Why would we want to do that?