Rim brakes don't work all that well in mud.
I was responding to a post talking about riding downhill in New England at 45 MPH, not mountain biking in the mud. That being said, my first mountain bike had rim brakes, as did all mountain bikes of the time, and my brakes worked fine, though disc brakes have the advantage there, and I have made no claims otherwise.
This thread is about Rivendell, and they build road bikes, not mountain bikes, and their bikes come with rim brakes. For that application, they are perfectly fine, and some may say they have an advantage. Disc brakes are great, but rim brakes work well, and have worked well for a very long time.