I have been doing the mountainbike thing for years, on sanctioned bike trails,
unmarked trails, mountainbike forbiddin, and crossing private land. But at the same time I followed the rules Gary pointed out. I've seen trails that mountainbikes have carved out 1ft deep trenches, and I've seen damaged trails from horses and motorbikes, never seen a damaged trail from hikers.
When it comes to who gets to use what trails, the basic rule of the land is what group has enough money to "volunteer" with the forest service, and in return the forest service gives them the access they want.