If you ever used wp, you know how you can convert a tag to a category or the other way at any time.
This often comes handy because a tag can turn out to be more important and used more often than you thought in the first place.Or the other way, a category may not have as much post as you foreseen, like one of your tags can have 25 posts, while a category has 3 where the conversion need comes.

Technically speaking, there is no clash or conflict, which doesnt make it entirely a necessity, but it would be helpful