One unmentioned feature of federation is that it lowers the cost of switching. If your provider begins to misbehave, you can jump to another one. This is especially true in email, where you can move from one host to another by updating your domain's MX records.
Without federation, switching means migrating both your host and your network of participants simultaneously. That is a much harder and more expensive coordination problem.
Without federation, switching means migrating both your host and your network of participants simultaneously. That is a much harder and more expensive coordination problem.