Nor with dirt bikes and snowmobiles. Those motorized devices are too heavy and move too fast to want to run w/o suspension.

I believe bicycles provide trade offs in this area that are either not feasible or don't matter with motorized vehicles. With a bicycle I can trade away suspension for less weight. I would never make that trade on a motorcycle, but on a bicycle it's feasible depending upon what trails I ride. That said, I have both a rigid bike and a quite nice suspension bike. I grab whichever I'm in the mood to grab. Sometimes it's one, and sometimes it's the other.