There are several modules that provide taxonomy term widgets that are more efficient at drilling down into large and complex vocabularies of terms, but I've not yet found something that just limits terms in some way.
The use case is somewhat like this: I have products that are classified by sport and by team. And teams can be rugby teams or football teams, and so on. Having one vocabulary per type of team feels somewhat weak, but putting them all in one vocabulary means the user has to wade through a lot of irrelevant terms.