There would be many cases in which it would be useful to filter based on the absence of a value rather than the presence. E.g. I would like to filter all tasks due on any date except this week. Or tasks assigned to anyone except unassigned or except "Joe". For instance I have a few dummy users to which we assign major tasks to filter them for gantt charts to keep the charts from having every last memo on them. I don't want those dummy user to show up in my weekly staff meeting.