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. It's fairly certain that the aerodynamic benefits of aero wheels designed for mountain biking are limited to fast races. Expect no gains in speed in normal off-road races, but worthwhile benefits of a percent or more as speeds get close to road speeds. And for downhill races, aero wheels seem certain to give you a healthy lead. More specific tests designed with mountain bike conditions need to be done to determine how pronounced the aero benefits are for these wheels.