That's hardly a big difference and doubtful as being the cause for much of a drag penalty, if any. Besides, the rims are more aero without the brake track, so makes up for it. And lighter too. I am about to have a set of Enve disc wheels built up and the (supposed) weight is the same as the non disc version despite having a greater rim depth.
Of course the rotor adds a little weight and little aero penalty, but still a non issue for most people
And the typical average speed in the races I do is 28 mph. That figure of 3 watts is for the most common conditions, not the more extreme conditions that decide races.
So is it a common conditions that you are you out in the wind the entire race, or are you drafting/in a peloton? These aero penalties are usually calculated in a wind tunnel that doesn't necessarily represent a race situation where you are surrounded by other riders
And lets not forget that the vast majority of cyclists don't race, your scenario doesn't apply to most people who are looking to get disc brakes
That average speed is rolling turns. So not all the time is on the front but a considerable amount still is. It's not a big difference (similar to the difference between a 808 and 404) but add it to an aero helmet, frame, handlebars, shoes, shaved legs, skin suit and it will add up to a noticeable difference.
I reckon most people choose carbon wheels for their looks. Add on disc brakes to get the braking back up to the level of an alloy braking surface and they may as well have chosen some FLO30 alloy wheels as the drag and weight would have to be pretty line ball.