Questions about the differences between a book and its film adaptation are usually quite distinctive and not like any other type of question on this site, needing a special approach in order to find answers to them. So from the beginning I'm tending towards thinking they should have their own tag.
Disclaimer: I also have a personal liking for this kind of question, although it's not one of my "favourited" tags, and I believe I have more posts in this tag than anyone else.
Going through the criteria in AncientSwordRage's answer, I found a few which I disagree with, and thought the differences were enough to be worth making a new answer.
(Tag scoring criteria, as ever, are copied from this post.)
- Does it make sense to be an "expert" in book-vs-movie?
No; score -1.
- Does it make sense for a question to be tagged only with book-vs-movie?
No (there should also be a tag for the film or franchise in question); score -1.
- Does book-vs-movie have a single, universally-unambiguous meaning?
Yes (the tag wiki description is clear enough); score 2.
- Is book-vs-movie likely to be used correctly just based on its name?
Yes ; score 2.
- Are there "enough" (> 15) but not "too many" (> 10% site-wide) questions that qualify for book-vs-movie?
Yes (there are 79 questions with this tag, and certainly not thousands that need it); score 2.
- Are people likely to use book-vs-movie to find questions to answer?
Maybe (this is one of my favourite tags, and I just went browsing through it to find a question to answer, so others might be doing the same); score 1.
- Are there likely some users (be objective!) who will favorite or ignore book-vs-movie?
Yes (the tag already has 6 "followers"); score 2.
- Could book-vs-movie be reasonably used to feed questions to a specialized chat room?
No; score -1.
- Can book-vs-movie be used to search for questions (for any reason) in a way that keyword searching cannot accomplish?
Yes; score 2.
Total is 8, which should be enough for the tag to be not worth destroying. From the linked question:
tags that score > 12 are "good" tags, tags that score < 8 are "terrible" tags, others are likely good but may need some clarification/renaming/etc.