Would that really help? And if so, is that all you would need?
I could be entirely wrong, of course, but I am guessing are using the tags to emulate "paper GTD" where each task can go, and must go, in one single context. In that case, I agree that being able to filter for tasks that have no tag would be a great help.
But if you are instead trying to economize with your tagging, and apply tags to only those tasks that are in some way unusual, such as "can only be done when it is raining", and/or if you can apply several tags to a single task, such as "Computer (because you need a computer to do it) and John (because you also need John) and Office (because you also need access to the archive there) then it will not really be of much help to be able to filter for "no tag at all".
A feature that could help even in these circumstances and would be immensely useful for other things as well would be "exclusion filtering" (NOT filtering) to eliminate tasks that have any of a number of tags that you do not want to see right now for example NOT Alex (sick today), NOT errand (cold outside) etc.