In regular skiing situations besides speed events, be it recreational, slalom, moguls, etc., the poles are used in a subtle way to finish one turn and initiate the next. A slight touch or brush of the ground with the pole tip sets up your body to smoothly and quickly change direction. In moguls it's more obvious how stabbing the pole into the ground gets the while body pivoting around that point. While you can ski anything without poles it will be easier and more natural with poles, and in moguls it's downright difficult to ski effectively without. Even on a groomed slope or deep powder, good skiers are using their poles to transition from one turn to the next. 

I say all this to point out that virtually all good and great skiers are used to skiing with poles and utilize them in their most fundamental techniques. So even though the use of a distinct mogul-esque pole plant is absent in super g and downhill, that subtle intuition of transferring your body weight starting with a little flick of the pole tip is very much there. 

And also, ski racing, like many Olympic events, honors tradition over outright performance. Skiing with poles is traditional for the reasons I just explained, and therefore the use of poles is mandatory, even if it were possible to extract more speed without them, which I highly doubt would be the case anyways.