I think defederating instances that post illegal content makes sense, as the admins are responsible for the content on their server.
I think defederating instances that host belligerent users and content makes sense, because it protects those that are targeted by them.
I don't think defederating instances that post content some people don't want to see makes sense, a la Lemmit. If the admins want to go that far, you might as well close off the entire fediverse and only host content at your personal approval. Dunno why you'd need a Lemmy instance for that.
My last point is talking about instances like Lemmit, hence why I mentioned it specifically.