There are two main purposes for tags:
- To aid search algorithms in distinguishing the most important words or phrases from among many;
- To organize information into categories that can be browsed or filtered.
Despite the word "case" appearing twice in the title and almost a dozen times in the body of the example question, a search for "mysql case" on SO does not yield this question among its first 1,000 results (I stopped looking after 20 pages).
A search for "case expression mysql" brings it up right away, and "case statement mysql" at least has it on the second page, but I would expect most people to start with "mysql case" and go from there. Not only that, this question clarifies an important syntax distinction that warrants elaborating on the official documentation.
This question and its accepted answer are high quality, and should be among the top results* in a search for "mysql case" - for that reason alone, I'd say it deserves the tag, provided this will change the behavior of the search algorithm. After adding the tag, this question and one of its answers immediately jumped to the first page of results for the same search query, which is strong evidence of the importance of proper tagging.
Therefore the tag should be added because it improves internal search results, even when the word already appears many times in the question title and body.
It's hard to say how many people would care to browse this particular tag, but until such time as the tag is burninated, it should be assumed to be a generally useful tag.
Therefore the tag should be added because without it, the question is not included in the category represented by this tag, and it clearly belongs in that category.
*Two things are noteworthy about the second result of this search (How does MySQL CASE work?): first, the question does have the
case tag, and also uses the word in title, question body, and answer; second, and more importantly, the accepted answer with a net vote score of +44 is actually wrong, in that the user asks about the case statement syntax and is given an answer that describes the case expression. Neither the asker nor the answerer of the question explicitly acknowledge the miscommunication, and unless you read the comments closely (or are already familiar with the distinction), you'll miss it. Yikes!