I mean, I keep coming back to that being the entire point of the Fediverse.
Sure, it means in some ways discussion can be different on different servers, but isn't that the entire point? To not have a single point of failure? If one instance puts down draconian rules that makes a certain community unwelcome, then why not move to an instance that you feel is welcoming?
In other words, post where you feel like, and/or where the people are. I really don't have an issue with cross-posting to other instances or having different discussions about the same subject.
Some instances have defederated with others, so another aspect of this is having multiple communities means people who have been defederated can still take part in the same community on a still federated instance.