That is true the hub acts as a spacer but it doesn't space the inner races in the same way. It would be like installing the pinion bearings with out the spacer or crush sleeve and using a nut with a cotter pin to set preload. I haven't ever done that to a rear end but something tells me it won't work. The principle is the same with the wheel bearing spacer. I'm not saying you can't drive thousands of trouble free miles with a conventional set up without a spacer. Obviously you can. It seems like the spacer set up has less drag when you spin it by hand. Maybe it's an illusion. I don't know.