Define racer?? I race road, XC, dual, and DH..... Rollers have always been to build a smooth pedal stroke which was most beneficial riding in a pack on the road but also helped on the mountain bike as well. That's not saying that there are or weren't more uses that I didn't know about. I figured roadie section, roadie answer.
I have not raced BMX but might try cruiser class out this year at a local track. I had buddies that raced BMX and not one of them owned a set of rollers. But hell.... what do I know.