I can understand that point of view. But on a 7 day cruise, as the formal nights are not on the first and last nights, it does mean that the two formal nights are squashed together, and subsequently loose their "exclusivity".
How about one per seven day cruise, and three per fourteen day cruise ? I know that will cause issues with those cruises sold as either 7 or 14 days [from Malta, Barbados, Tenerife], but it might be a workable compromise.