I agree with your statment
I have a bunch of topics in different categories that I want to move into a single category. Before I move them, I want to tag them all with the old category name so I know where they came from after they are moved.
Also, at some point, I'd like to be able to bulk move topics based on their tags. This would be done if a category got too large/busy, and it warranted splitting out certain things into a new category.
Is any of this possible?