Mountain bikers cannot be controlled. They say that they will educate their peers, and make them obey the rules, but their peers ignore them, cursing them and showing them via sign language what they think of the rules. Their helmets, goggles, and other gear hide their faces and make it impossible to identify them. They travel so fast, that they can be caught only by a motorcycle, increasing the expense and impact of enforcement. Bike patrols are ineffective, and increase impacts still further.
Rampant erosion is only the most visible sign of mountain bike damage.
Probably their worst harm is caused by the ease with which they allow people to get into wildlife habitat areas, greatly increasing human presence and human impacts on our precious remaining habitat, and driving out most of the wildlife. Mountain bikers have also killed many small animals, which they travel too fast to avoid, included the federally Threatened Alameda whipsnake. Most discussions of mountain biking ignore wildlife, and pretend that the various human user groups are all that matters. Similarly, mountain bikers intimidate and endanger equestrians, children, the elderly, and other hikers, and have already driven many of them permanently off of their favorite trails. At the very least, meeting a big piece of machinery in the wild, when you are there specifically to relax and get away from the headaches of city life, destroys your experience.