My tourer has discs, and I love them.
The basic problem with discs for touring is that they do get in the way of most regular racks -so if you have front discs, they will more than likely get in the way of any front low rider rack you wish to use.
Other disadvantages? Well if you do need spares, a bike store might not have them compared to "regular" cantis or V's -though ask yourself when you last had trouble with a good quality brake anyway? The biggest disadvantage to me though is that given a good quality canti or v-brake is usually more than sufficient, the amount of money required to get a disc brake gives low bang for the buck. Having said that, my disc brakes have been the best brakes I've had, and since I've already spent the money, I'm not giving them up any time soon!