It's often the hills that slow you down, not just the next technical bit that you have to brake for, so I would guess that the loss of time due to the weight of aero wheels is maybe 0. I find it hard to believe, but there it is. In spite of calculations by my consultant physicist that prove the insignificance of the weight argument, many off-road racers who depend on their ability to climb will not tolerate an increase of any magnitude. A final consideration is the durability and cost of aero wheels. The rigors of racing off-road increases the potential for broken components.
Aerodynamic wheels are considerably more expensive than their spoked counterparts.
So the Spinergy wheels would have to last twice as long to be cost effective. Unfortunately, it is not possible to true most aero wheels, so the lifespan may be shorter, especially if the rider uses them for downhill racing. Ultimately, cyclists will decide if aero wheels are worthwhile. Most reviewers do agree that the wheels flex considerably when riding. Some riders feel that this helps provide a cushion effect which lets them ride longer with less fatigue, while others feel that this motion is disconcerting and actually saps energy.