Good quality mountain bikes have rims, hubs and frames built to take more stress than the bike is likely to get from riding on a paved road. For example, a "middle level" Trek mountain bike is engineered for the possibility that a 200 pound (plus) rider might actually race it down the side of a mountain. Most of us will use that same bike to ride to Starbucks, but it really can safely carry you down a mountain.
A mountain bike equipped with slick tires, such as the Specialized Fatboy, makes a fine road bike for everyday use. Make sure the bike is carefully matched to your height, and leg length - a correctly fitted bike is far more enjoyable to ride.
Some folks may claim a mountain bike makes you work a little harder than a 19 pound road bike. Assuming that is true, couldn't most of us benefit from working a little harder while we are enjoying our bikes?