People will remember slipped dates but not slipped features. If deliveries are made to meet deadlines, even if the deliveries do not include all the promised functionality you are less likely to get into trouble than if you had delivered all the functionality but at later date. If you regularly deliver production quality code on promised dates end users and customers will tend to be forgiving, even if some planned features and bug fixes are not delivered on that date. For some end users, getting anything on a regular basis, and not necessarily everything, can leave them delighted.