I don't think it's completely useless, particularly if you're answering a question about the cancellation of a show. Using the above (MattD) answer, we can look at Family Guy, which I think has been cancelled twice.
A tag with just Family Guy will draw plenty of eyes, whereas a 2nd tag of Cancellation will most likely draw in the eyes that actually know about the cancellations and reasons. The question won't get lost in the sea of Family Guy questions, and the Cancellation tag will help it stick out. Since the answer is going to be known by a smaller percentage of people, I think it's more helpful to use the tag.
I know people freak out about tags sometimes, but I look at them as Google words; when I google I pick 3 or 4 words that are the most important because I know if an article or webpage has those words, it's probably what I want. I weed out the chaffe that way. I look at tags somewhat similarly; a few pertinent tags can get right to the question you're looking for.
Keep in mind, after they pass by the "Hot Topic of the Day", they're still on the site and someone may come along 2 years later searching for an answer when that question is buried on Page 80.