django@social.coop yes, that could be one use case, but what I want to stress is that a topic belonging to a category should stay in that category.
For example, if a topic "PieFed is great" is started on fediverse@piefed.social, it wouldn't be right (nor would I really be able to) move that topic to fediverse@lemmy.world — I don't own either of them!
So to de-duplicate topics or move them around based on my personal liking is possible, but goes against the reality that a specific topic is a member of a specific community.
It's more a restriction out of principle than anything.
Now, if one were to put together a proper cross-posting FEP and it were adopted by the threadiverse implementors, then we'd really be cookin'.