I think it's a good thing, because it discourages centralisation. If we end up with a bunch of specialist instances, then diversity suffers, because everyone looking for a specific area will end up on the one single specialist instance dedicated to it.
And I say that as the admin of an instance focused on the trans and gender diverse folk. There is a reason that we don't enforce specialisation on those topics in our instance communities though. Even so, we still tend to be "the trans instance", when I'd much prefer it if we were just one of many, like we are on the microblog part of the fediverse.