I think that untagged is very useful, as it is a clear indication that a question is in need of some attention. The specific attention that most of the questions that have the tag need is burnination -- even the ones that are open are crap questions (I have no idea how that "R" question is still alive).
I don't think that it is an issue of people using the tag (you cannot create a question without any tags, nor can you explicitly specify untagged when creating or editing a question; I just tried it and the system told me off). It does, on the other hand, make the place look a bit untidy and unmaintained.
If some kind souls want to start weeding the garden, and let me know which questions need shiny diamond powers, I'll help out where needed.
A tag is a keyword or label that categorizes your question with other, similar questions.. How does
untaggeddo that right now?