It just ensures that the recipricating parts are equally balanced. Most drag racers I know of use a full forged crank that has not been knife edged because they want the strength that the mass brings and they do not use an under drive pulley. As far as I know, you don't spin balance a crank. It is done with plumbs that represent the weight of the rods and pistons. Therefor you don't really know what the effects are on the harmonics. Taking away much of the mass of the crank in order to knife edge it is not a good idea in my opinion.
