Brakes are the most troublesome part of the component search for touring bikes due to the difficulty of matching road levers to various styles of frame and long reach powerful brake. So whatever the faults of discs it isn't as though all the other options are easy in every case either.
Generally the discussion starts and ends with the cable actuated version of disc brakes. For better or worse there seems to be an agreement that a hydraulic brake systems on a touring bikes is too complicated for easy field service. Also, most touring bikes do not encounter the kind of mud that stops rim caliper brakes dead in their tracks. So two of the main advantages of "discs" die before we even start the discussion.
- Wheel separation issues should be dealt with, not used as an excuse for not mounting discs. I wouldn't by choice have quick releases on any touring bike. My mom's touring bike had a nice compromise, solid axles with butterfly nuts on them. Another option would be pullable axles as on motorcycles and enclosed drops. Lawyer lips may also helps, so possibly uber bike makers/users should stop the name calling.
- Discs do weigh more, and the forks they run weigh more. Fully loaded touring bikes probably should be dropping the wimpy forks anyway. The fiddly tapered tubing in a lot of touring bikes is a nice vintage look but just as heavy as straighter stronger forks (the thin ends are swaged and have near the same metal in them). The idea there is a lot of useful compliance happening is scary if anything, on a touring bike, that is the job of the tires.
- Easy to adjust, and stay adjusted if the rim wows, better than the rest in this regard.
- Run with either kind of levers required for drops or bars, though you need the right caliper.
- Discs can be fragile, but the are also light and easy to replace, and they are easily found in bike shops in the first world.
- There are rack mounting problems, but they are only integrative. There is no serious difficulty if the discs are designed in as part of the package from the beginning.
- Wheel strength problems are twofold, one is the tweak put on the spokes when stopping, and the other is the need to dish the wheel on the left side also, wherever there is a brake mounted. It's not good news, but it can be dealt with, by solid building technique, and also by future potential mods like a wider than 100 front hubs.
- Disc systems aren't particularly expensive, but they require an added BO on the frame, and also the Avid brake itself. You can pick them up for under 50 per wheel at times, and that is cheap compared to premium rim brakes, but very expensive compared to the kind of tripe that gets fitted to a stock touring bikes, and might cost 10 bucks a pair of brakes.
- It probably is true that discs are not super familiar among tourist. Though for anyone who follows cycling, disc brakes are one of a handful of big gets the cycling industry worked on for a decade, and every tiny improvement was much written about in the mags. Think of discs; hydraulic brakes of any kind; suspension; better shifting derailleur and clusters; aero wheels, etc... Maybe today, tubeless tires.
- Discs are ugly. They have a lot of nooks that catch dirt. There isn't anything really clean in the way of an install like an Exile sprotor mounted, drive side brake on a Harley. Imagine if they had something like that and the hydro was wired through the frame for a cleaner look.
- Some of these issues go away if you mount only a front disc, or a front disc and second brake system. How likely would you be to pop off the front wheel if it was only 50% of your braking force? How serious a problem would hydraulic reparability be if you could fall back on a second brake?
- The final problem with discs is that calipers can be made to work, and they are rather beautiful to boot, cheap, and accounted for on most frames. If it ain't broke don't fix it. I think that is a fair attitude, as long as one keeps an open mind to the circumstances in which rim brakes are a somewhat limited system.
I think in taking anecdotes about stuff like bikes flipping (gotta be a severe downhill in the woods somewhere). Or Thorn's caution on discs, one should consider the source. If you sell bikes with touring style forks and outfit them with flat bars and MTB discs, stuff may happen. I don't think that package of ideas is going to travel well to every bike.