Pros:

• You get Uncle JimBob's boomer meme posts in your Lemmy feed now

Cons:

• FaceMeta suddenly represents a huge percentage of the userbase for ActivityPub, starts adding "extensions" that only work through Threads, promotes "free communication" between those on Threads and those elsewhere, then slams the door on everyone else causing fragmentation in the space and hopefully (from their perspective) killing the protocol.

At least, that's more or less what happened with XMPP. I'm not entirely convinced that Threads federation will lead to the same fate, but the idea of having Facebook shit in my feeds is not an appealing prospect in the first place.