Well, reduced drag is always reduced drag, regardless of speed. Whether you think the reduction is of benefit is a personal decision. Most sporting riders will see the potential to gain more speed for a given effort level, or to maintain a given speed for a certain reduction in effort, to be a benefit.

FLO Cycling has shown that drag reducing benefits accrue at all speeds given sufficient distance, and that more time is saved by reducing drag the slower one covers that distance: FLO Cycling - Cycling Wheel Aerodynamics - How Speed, Time, and Power are Affected by Reducing Drag. Of course, that's not how sporting riders measure benefit, though, as most of us want to go faster.

Another example of benefit can be seen in this Cycling Weekly video where "benefit" was demonstrated over just a constant effort of 10 minutes duration at a commonly produced 200 watts: How much faster are aero wheels? (video) - Cycling Weekly