Mountain bikes are allowed in many local non-UCI cyclocross races. I think this is great becuase it brings more people into the sport. Unfortunately, around here the poor folks on the mountain bikes are often the ones who end up being lapped by the field. Either these people are slow, or more likely, they're at a disadvantage because of their bikes. Most, but not all, cross race courses favor a bike with the ability to go fast. The technical or rough areas where a mountain bike would be an advantage are minimal, and are more than compensated with fast sections.