Let's say, for the sake of argument, that you are building something for someone. You tell them, "It will take us X amount of time to build the thing." They then proceed to take every opportunity to complaing bitterly to anyone who will listen about the fact that it is going to take you X amount of time to build the thing.
Call me crazy, but I have trouble with the fact that you then proceed to report the status of building this thing as green or on track, even if every indicator points to the fact that it will actually be complete in X amount of time.
And no, Agile doesn't solve this problem.