Why couldn't an inside neck reamer be used to remove it? Does that cause a problem?
An inside reamer can easily remove the doughnut. But that doesn't mean it will leave the neck the same thickness all the way around, partly because with a lot of "standard" mass-produced brass the necks tend not to have the same thickness. This can definitely cause accuracy problems.
Outside-turning not only remove the doughnut but insures even neck thickness.
