rhm beat me to it! Not many mountain bikes (in my area) have actually been off the pavement. I think they are great for a utility bike or a fire-road/2-track rider. Camping is pretty big in Michigan, and the older mountain bikes are perfect for gravel roads.

I have converted a lot of older mountain bikes to road-oriented tires for quite a few people. They like the fatter tires than what's on most road bikes. Less likely to ruin a tire or rim on a pot hole or curb. The most technical thing most of them will ever do is ride across the grass somewhere.

Back when mountain bikes started selling like hotcakes, there wasn't a huge choice. I think people liked the idea of a mountain bike, without the actual mountains. If you wanted a fat tire bike years ago, you had to get something like the Schwinn heavy duty, which weighed like 50 pounds. Mountain bikes bridged the gap, in my opinion.