No one should federate with threads for a very simple reason: they are a known entity with a horrible track record for moderation. If Facebook had always been part of fedi, they would already be widely blocked for failure to moderate bigotry and harassment, and failure to live up to the mastodon covenant or any similar fedi standards.
Federating with threads doesn't just mean more content for everyone, it means having moderation work offloaded onto fedi.