It would be useful for sites that have dedicated categories (more likely subcategories) for things like events, galleries, products, press releases, support, geographic areas, languages, etc.
In my specific case I have different groups that each have their own event and gallery subcategories. For users who participate in multiple groups, it’s a way for them to see everything under one tag.
This would be away to easily aggregate topics across a site and reduce admin work. Thinking it through more, it probably should just be the first post in a category, not every reply, that gets the tag.
Not a show stopper, but might be nice for some sites.