Which is another thing that just does not compute in my mind as a concept, much like pair programming. Probably for the same reasons. That said, it's obviously very situational and personality-dependent.
When it's working I think it's absolutely great. Together we find the best solution in the fastest time with less errors along the way. I'm also less sloppy and more focused when someone constantly sees my work and vice versa. At the end of the day you can celebrate what you've done, both feel responsible for the work and knows exactly how it works.
The problem is you need to fully trust your collaborators, and that's really hard to find in this industry. Most people never find it. I haven't.
I've seen it though. It exists. Very rare. I know two guys who used to participate in Ludum Dare over 10 years ago who would have loved something like Zed if it existed back then.
By contrast, at your 9-5 where you don't know if your coworker is gonna stab you in the back for a promotion (which I suspect is most people in the comments here) then Zed is probably not a great tool. Git would be a better fit for that. After all it was made to develop Linux, which is one of the lowest-trust collaborative environments out there.