While I agree that mountain bikes can be versatile, one must keep in mind that the same mountain bike that can easily handle extremely rough terrain would absolutely suck as a commuter. A fully rigid mountain bike, which would make a decent commuter, isn't particularly well suited for very rough terrain. Some mountain bikes are useful in a variety of environments, but are not well suited to do anything extreme. Others, such as a downhill bike, aren't very useful off the trail.

I'm a year-round commuter, and my only bike is a fully rigid mountain bike. But, I would be naïve to believe that the platform is not without it's limitations. Pretty much anywhere off-road that my bike will go (such as a rock garden like the one pictured in the OP), I could just as easily ride a hybrid. A cyclocross bike would be just as good in most cases, almost as good in a few cases, and better in a few cases. For long distance rides, a road bike, touring bike, or even a cross bike would be much better.