Most off-road events are a lot slower. At that speed, aero wheels simply won't help. Speeds for some events are increasing. Recently there has been a proliferation in the number of off-road stage races. At that speed, aero wheels could be a small benefit.
Downhill events show much more promise for aero wheels.
At speeds this high, aerodynamics play a very important role. Weight is crucial in cycling. In spite of the availability of full suspension bikes, nearly all professional riders still choose to ride on hardtails, which have only front suspension. A full suspension bike weighs too much. Although Spinergy does not claim weight for their rear wheel, they do mention that their front wheel weighs 895 grams.