The concept of allowing users to write descriptions for tags presupposes that there exists people who are somewhat knowledgeable about the subject of the tag, and that some of them will be motivated enough to take the time to create and edit these descriptions.
It can, therefore, be said both about persons who follow tags and persons who edit tag wikis that
- They care about the subject the tag represents
- They know at least a little bit about the subject
If the same people who would follow a tag are the same people who would create and edit tag wikis, then we should try to motivate users trying to follow a wikiless tag to create that tag wiki.
I suppose there is the nice way to do this,
We see you are following the [Derpify] tag. This tag does not have a wiki. Would you consider taking a minute or two to create one? Please click here to create a wiki for this tag
Or you could be an ass about it
Sorry, you cannot follow a tag that does not have a wiki. If you wish to follow this tag, please click here to create a wiki for this tag. This will not only allow you to follow this tag, but will help drive more interest to questions with this tag.