I recommend deletion. It seems that that tag is there just as a way to avoid using an actual tag.
Since tagging is a way of adding meaningful information to our questions and we have plenty of tags, I can't think of a reason why a question shouldn't be tagged or couldn't be tagged (if the latter is the is, a new tag can always be created or suggested, or the question even retagged).
All the questions that seem to use the "unttaged" tag could actually use another of the meaningful tags that we have. So, keeping the tag could only cause a detriment in the quality of the questions or the information search mechanism of the site.
Edit
After seeing Flimzy's comment, I thing we should rename the tag as no-tag. That will do in both languages and will also keep the tag for the uses Flimzy is describing.