You are going to hear arguments on both sides of the fence on this one.
Some guys swear that if you super-inflate your MNT bike tires, the rolling resistance is not much worse than a road tire.
For me, it is all about ease of riding and MNT bikes just seem to take more energy than road bikes. Plus, my hands go numb and my back gets a little sore from the limited positions provided by mountain bike geometry and the typical straight handlebars of a mountain bike. Remember that touring usually means spending at least eight hours a day in the saddle.
I can tell you this for certain; I organize a number of group rides and tours. With the exception of the trememdously fit riders, most of the mountain bikes fall to the back of the line - and some of them WAaaay in the back of the line.
My suggestion would be to go with a solid touring road bike rather than a mountain bike. If, however, you are in a money pinch or you simply love your MTN bike, you could certainly tour on a mountain bike.