A great example of why this would be useful is in the first page of the books and literature section
WOT Gathering StormThere are only very very minor spoilers in here, but with tags the discussions could be more in depth and such.
Perhaps those who had already read the book were reluctant to post plot lines and such (I thank you greatly for this ^_^) but with tags those who had read the book could discuss it in depth, without resorting to PMs and the like, while those who were interested in the book, but didn't want to be spoiled could still browse the topic.