I'm building a bike with disc brakes and cantis on the front wheel, and cantis on the rear wheel. This is one of those, "I had an experience builds", which are always a lot of fun. I could certainly see having a mix of brakes on a touring bike. There are plenty of advantage, and I really don't see the parts problem. The BB7 brake has less maintenance issues than my straddle cables that require two wrenches. Different brake systems are a comon idea on bikes, as are three brake systems. It's a sad fact that my touring bike is trotting along with as much weight abord as the average tanden, so some redundant braking makes sense.
I think that discs on the back make sense on MTBs where stuff like mud is a main motivation. But that isn't a problem on my touring bike, and rim brakes will hold the rear wheel easily. Were the front wheel needs extra braking power, having two different systems on the bike allows stopping power redundency, a blend of performance and wheel loading. Sure it is a little heavy, but a lot less so than Rohloff or Big Dummy.
My own build is sorta stuck since while I have the hub, brake, and fork, there are other options that don't call for a dished front XT wheel build. Probably will happen, but not certain to make the cut. Another option is two rim brakes up front.