I always take them off, but today discovered a use for them. Descending a rocky hill, my front tire kicked up a rock (happens sometimes; stubs a toe or the rock ends up bouncing around the front triangle, even landing in a bottle cage!), this one found its way INTO my rear wheel spokes, next to the hub shell. It jammed the aluminum cassette spider from the inside and before I knew what was happening three spokes went bang. This was 12 miles into a 50 mile epic, literally middle of nowhere. Of course, I didn't have a spoke wrench (!), but the wheel was still rideable barely, and I made it back before the frame wore a hole in the tire sidewall. I think the dork disc would have prevented this.