Adam Weinroth asked Clay and I five questions about tagging in an email interview posted here.

As for uniform tagging, that can only work in situations where there is enough force to expend making the users behave uniformly, and where it is worth expending that force. For example, the Diagnostic and Statistical Manual, (DSM-IV) used by American psychiatrists and psychologists, provides a relatively standard way to diagnose mental disorders. It only works as well as it does, however, and that not perfectly, because it is produced by the American Psychiatric Association, which body can exert considerable force over its members, and DSM-IV is only to be used by the members.

The amount of human cost, in other words, in creating an enforcing uniformity is so high that attempts at such uniformity will fail in most cases. Fortunately, tagging allows for degenerate cases such as alternate spellings and phrasing. This would be a problem if there were only one tagger, responsible for a large group of users, but with every user a tagger, the loss stemming from degenerate cases quickly shrinks, while the value from multiple points of view grows.