Precisely, they are designed for racing, and racers want every performance edge they can get which means as little rotating mass as possible. They are made to run on a smooth flat surface, with no obstructions. One of my friends had one of his MT fronts go down driving back from a run...it happened in the pits (thank goodness), ran over a hard piece of plastic (which is why I mentioned "plastic" a few posts back) and the tire went down INSTANTLY. He ended up having to pull out of comp because he didn't have a spare.