Well, I thought of a possible use case for tags in a forum. Be patient, I have to set it up.
Say there's a forum that rules with an iron hand. Everytime someone asks a topic that has already been asked, they are tersely told to search the forums. Problem is, the person posts the topic because they already did a search of the forums, and all they could find were these terse responses saying to search the forums.
Suppose further the search engine can be tweaked so that if a thread is tagged with a certain tag, it gets the bottom priority in search results or is ignored completely. These "search the forum" threads could be tagged by the admin for that purpose, so that the n00b could actually find what he is looking for by searching the forums.
This case doesn't seem to apply to the people-friendly and search-friendly DonationCoder forum, but I wanted to give an idea of when it might be useful.