Rip,
I've never heard that about the Phantom, though I'm certainly no pilot or expert on her. But I have heard that the nose can yaw around a teeny bit at high speed (especially if you lose the Stab Aug system) and not sure I wouldn't want rudder input available. Also, due to the way the aircraft rolls, where one wing uses a spoiler to kill the lift over the wing, it would cause somewhat of a yawing moment in turns and again, would probably want rudder to be effective. Are you sure you didn't get this backwards? At slow speeds and/or high AOA, you do not want to roll with aeleron/spoiler due to this yawing tendency which could put you into a nasty spin but instead used only rudder input.