While I don't think we should get into the whole "hierarchical tags" type of thing, I do think coarse-grained + fine-grained are both useful.
Taking a different example, halacha, hilchot-shabbat, and shabbat.
Sure, we could just tag it with
shabbat... but that's not really the same thing, and
hilchot shabbat is a big enough subject of itself to merit its own tag. It also makes sense to encourage that level of specification...
No, it's not the same thing as shabbat, since there are plenty of questions and topics to discuss about Shabbat that are not hilchati in nature.
Also, if we were to just use the specific tag, and use only hilchot-shabbat, this removes those questions from the larger set of Shabbat and Halacha. This makes a difference, e.g. if you have a posek (or anybody with a halachic interest in the site) that is interested in all types of halacha questions to the exclusion of e.g. aggada or lamdanut questions. In the same manner, historians may be interested only in history, or high-school Tanach teachers (or Machon Herzog ;) ) interested in all tanach-scripture-bible questions.
On the other side, you might have someone interested in learning anything they can about Shabbat - halachically, historically, experientially...
Hence, I would see all 3 tags (halacha, hilchot-shabbat, shabbat) as fitting, since you might be approaching that area (category+topic) from either direction.