There is no way that every passenger who carried on one piece of maximum allowable size could find a home for their luggage on most of NW's fleet. On the DC-10, for instance, with 9 across seating and very small overheads on the side, I think that four rollaboards would take up the entire overhead space for the row. Add the problems of bulkheads/exit rows where no under the seat storage is available and the overhead space used for oxygen, etc and there's no way it would all fit.

Besides, if the airline sells more tickets than seats regularly, why shouldn't they "allow" more luggage than space.

Just my .02