I won’t defend Meta but I don’t see the advantage of defederating when Threads activates ActivityPub.
By federating, we have access to all the people and content on Threads, removing a major barrier to entry for Mastodon/the fediverse for casual users. And because our accounts aren’t on Meta servers and we don’t need to install the Threads app, we aren’t subjected to the same level of privacy issues, at least not any more than what Meta would get by scraping our posts on the web in general.