As far as I know, it's primarily about getting the gearing down low enough for where many tourists prefer their gearing to be.

With a "road triple" you have chain rings of 50-39-30 and with a current 10-speed rear cassette you'll have something like a 12-27 in the rear.

With a "mountain bike" drive train you can get a triple with (for example) 46-34-24, or 44-32-22 and a 9-speed rear cassette of 11-32, 11-34, or even recently 12-36.

The road triple w/a road cassette in the rear gets you a low of 30 gear inches; the MTB drive drain gets you down to less than 20 gear inches. Big difference.

Take a look at the way that Bruce Gordon spec's his loaded touring bikes:
http://www.bgcycles.com/faq.html

Or Co-Motion Americano:
http://www.co-motion.com/single_bikes/americano.html

Nothing says you have to do a MTB drive-train -- plenty of people seem to survive with the higher gears. My own light touring bike is a mix-and-match -- a road triple in front (50-39-30) with a 9-speed MTB cassette in the rear (12-34). You can also do what Cannondale does on one of their bikes -- mix a "road" triple with an IRD wide-range 10-speed derailleur. But if you want low, low gears and are building a bike from scratch a MTB drive train will get you there faster.

The other common MTB component brought over to touring bikes is rear hubs with 135mm spacing. Supposedly helps create stronger wheels than a standard road hub.