I think such overly broad tags like
identify-this-movie are not more or less useful than overly narrow tags, like
dark-knight-rises. In the end a question can have multiple tags and one of them being a rather general tag (that usually describes the essence of the question pretty well) doesn't do any harm. In the end we got 5 tag slots to fill with useful stuff and they shouldn't just contain movie or director names but should range from broad to narrow.
A question should have as many (fitting, of course) tags as possible to gain as much information about it as possible. And if 30% of questions are tagged
plot-explanation, this probably means that, well, 30% of the question on the site are plot explanation questions. It doesn't need to mean that the tag is too broad. More often than not a
plot-explanation tagged question is indeed, well, a plot explanation question (likewise for
analysis) and that describes its essence pretty well, apart from any specific movie tags (which are there, too, anyway). If you're interrested in
plot-explanation questions, well, go search them.
You wouldn't dare to delete the
identify-this-movie tag just because 30% of the questions have it, would you (beware, I'm talking about the tag, not the question category itself, which is a hot topic on its own)?
EDIT: By the way,
plot-explanation is a totally different tag than
EDIT: In the end I think the plot-explanation and analysis questions make up the best and most important questions on the site and should be the signboard of this site. Once those tags are removed, how would you e.g. differentiate "how did Ethan know Jim was the mole?" and "was the usage of Job a symbol for Ethan's desperate situation?" from "wasn't it a bit dumb from Jim to steal the bible, sounds like a mere plot-device?" and "what edition of the bible was used in this scene?" if they're all only tagged
mission-impossible (or maybe, even worse,