I've toured on a couple of mtbs with no ill effect. However, these were older early-mid 1990's bikes with no rear or front suspension, so they had more of a traditional road bike geometry with longer chainstays (as opposed to the newer mtbs now which have geometries that take into account front shocks). I'd certainly not hesitate touring on an old style mtb -no problem. I would not recommend front suspension as it sucks up energy, and certainly much less a full suspension (let alone the problems associated with affixing racks). I currently use my backup tourer (90's Specialized Hotrock) as a commuter bike complete with racks.
So to summarize: yes, it's a good idea if it's an older style mtb with no suspension and a more relaxed geometry, you don't have to spend too much money to get it to where you need it to be, and you don't want to spend the money on a purpose tourer.
Advantages:
-cheap (really cheap to get an old mtb if you keep your eyes open)
-usually an easy way into touring; change the tires and tubes, slap on a rack, and you can be good to go!
-gearing is ready for touring. Mtb gearing is much lower than traditional road gearing, and for touring carrying a big load, that's good!
-some of the brakes on the mtbs were first class (good cantis or v's)
-assuming you use the flatbar or butterflies, no need to change shifters
-assuming you get a good example, the drivechain is inexpensive to replace parts.
-usually tough -remember, these were supposed to be mtbs
Disadvantages:
-not a "proper" touring bike
-may not have touring specific stuff on the frame like spare spoke holders
-if you want to use drops, then the cost of changing over can be expensive (E.g. new bars, new brake levers with possibly new brakes, new shifters, probably a new stem and new front derailleur if you wish to use STI)
-doesn't have higher gearing
-can be heavier
-may need a heavier duty wheelset if you need 36h as most mtbs come with 32h wheelsets.
-may end up spending as much as a proper tourer in an attempt to get it to where you want it to be (don't ask)