Much as I love Grant/Rivendell, this is one of the things they get wrong. Surfaces that start uneven won't necessarily wear down to be even. They will if you're using some big precision machining rig, but not on a bike. It's like saying that if you sand a whole surface evenly, eventually it will be flat. Anyone who has done any carpentry or woodworking knows this ain't the way it goes. On a bike rim it probably doesn't ultimately make much difference, but still, this falls into the category of stuff where no engineering knowledge is harmed in the making of the conventional bike wisdom.