The most compelling counterargument I've seen is made in the link below.

The core point is that even if I block Threads, (1) users there can still see responses to me (and therefore learn my username/instance), and (2) users there can see my public or unlisted posts if a non-blocked user boosts them.

I don't want to minimize this. But I don't think this creates fundamentally worse risks than currently exist. Unless you go private, anyone can find your profile & posts.

https://privacy.thenexus.today/just-blocking-threads-isnt-enough/

The problem with Threads isn't that it could federate with Mastodon.

The problem with Threads is Threads. Or specifically, Meta.

That problem exists whether federation happens or not, and to a large extent whether you block them or not.