In some parts of the world, you'd have trouble getting your hands on 700c tires, let alone disk brake parts. When I had an accident in central China I had to order a new wheel by mail. Then I had to ride 90km on a split rim with a towel inside my tire to the nearest town with mail delivery so I could pick it up.
Get rim brakes for a touring bike. Save the disks for cyclocross.
I was wondering this myself. Discs are getting pretty common in North America, but still a speciality item.