Road biking is built for speedy long distance travel, and very often other faster and bigger vehicles on the road influence your ride, putting you at risk. There are no such interruptions in mountain biking, and if you do have an accident, it is most often due to your own unpreparedness or overconfidence. Because you are more responsible for yourself, the probability of staying safe rises with mountain bikes.
Even though road biking does contribute towards one's fitness, mountain biking gives you an added advantage. Since mountain biking is done away from urban areas, the chance of one getting affected by vehicular or industrial pollutants is far lesser, and consequently this keeps you healthier than road biking, which is sometimes done even in heavy traffic.
Road biking is good for cardiovascular fitness as you pedal at a high rate for long periods of time. However, mountain biking is much more dynamic in nature if done properly, and requires one to constantly switch between quick bursts of energy and longer cardiovascular workouts, which involves the usage of many more muscle groups.
Whether it is a crash into a tree or a wipe-out in the dirt, mountain biking accidents usually hurt lesser than a car crash or a fall on the pavement. Of course, the exception to this are major accidents such as a wipe-out over a 30 foot cliff. Also, the bikes are tougher, hence have lesser chances of getting damaged.
By its nature, mountain biking is a lot about exploration and a certain amount of risk taking. Bringing this activity into your life automatically makes one a more interesting person, where you have tales of climbing treacherous terrain, avoiding the odd wild animal, and the triumphant but dirt and blood-covered return to civilization.
If you are a well-prepared traveler, mountain biking allows you to explore different places off the beaten track, through jungles and deserts, no matter how remote they are. The likelihood of other tourists reaching here is minuscule, and you can connect with the locals much more closely. If you're lucky, you might even be able to find a hidden wonder within your own city.
Within the urban environs of convenience and comfort, one can easily get complacent. However, by adding mountain biking to the mix, the feeling of being able to get through a day expedition solely relying on yourself is very satisfying. Also, knowing how to take care of mishaps, means that you might be able to help some poor soul who doesn't, which is even better.