There are a few concerns with disk brakes on touring bikes.
First, to accomodate the rotor, the hub flange spacing
must be narrower. This will result in a wheel which is not
as strong. Additionally, the front wheel will have dish.

Second, the frame and fork need to be beefy to handle the
torque from the brake. On the rear this just means extra
weight. However, on the front, it also means a very stiff
fork which will adversely affect ride quality.

Third, it is difficult, but not impossible, to fit racks
around many disk calipers.

Fourth, there is evidence to suggest that a front disk brake
can force a wheel from the dropouts unless there is some
sort of secondary retention device (like lawyer lips) or
other provision to prevent this.

Fifth, there has also been some evidence that rim brakes can
generate more sustained braking power and dissipate more
heat prior to failure than most disk brakes. Rim brakes
fail from excessive heat by blowing a tire off of the rim.
Disk brakes fail frome excessive heat in a variety of ways:
boiling fluid, warped rotors, melted plastic parts, glazed
pads, etc. Generally speaking, rim brakes will tolerate a
higher level of heat than most disk brakes. The best disk
brakes in this regard are those with heavy, large diameter
rotors. These can be superior to rim brakes in heat
dissipation.

The main advantage of disks in mountain biking is
performance in muddy conditions when rim brakes would be
packed with mud. This generally isn't an issue for touring
bikes (unless off-road touring). Both rim and disk brakes
need to clear the braking surface of water before effective
braking can begin so there is no clear advantage for either
in the rain.

Todd Kuzma
Heron Bicycles
Tullio's Big Dog Cyclery
LaSalle, Il 815-223-1776
http://www.heronbicycles.com
http://www.tullios.com