Based on data from https://fedipact.veganism.social/ it seems that the majority of instances blocks threads.net. I’m sure there’s Lemmy instances with either approach that have slipped through the cracks as the list is a work in progress.
The percentage of users doesn’t correlate to instances as the biggest instance on the Lemmyverse has roughly 3x the users of the second largest, a NSFW instance, 4x the users of the biggest “niché” instance and 5x the users of what I see as the second largest general purpose instance.
There might be no benefit to you but the people who would be the direct targets of every hate group like LibsOfTikTok currently active on Threads probably feel a bit differently.
This is in no way a knee-jerk reaction to a hypothetical alarm. Meta have directly contributed to a genocide and allow hate groups to grow unchecked on Threads. It blows my mind that the fediverse - itself built as a series of alternatives to shitty companies doing shitty things - thinks the harm Meta will do is hypothetical and unproven.
The benefits to not federating are many, but the best one, in my opinion, is not exposing current fediverse users who will be targeted to those who will target them.