i disagree with your logic. first, most urban riding very closely replicates the skills used on a mountain bike on a trail, like creating your own line, negating obstacles, etc. bmx dirt jumpers and park riders require very highly prepared surfaces, because their bikes are so specialized to prepared terrain, they dont have the ablility to ride jumps without transitions, drops, etc. they are also dismal bikes to ride on a trail, or commute on. urban mtb's can ride much more varied and rough terrain, creating jumps out of random junk hanging around, making their own lines in a city that often dont include any transitions, using stairs, ledges, etc. i can and have ride a trail with an urban bike, as well as commute to class on it.