Originally you would get away with what Richard described, more so if there were at least 3 other clear stopovers on the ticket, but then several grey areas were opening up on ticket re-issues, sidestepping the intent of the rule etc. It was made clear that a stopover on *RTW's would follow IATA general rules, clearing up many issues immediately. One major concern was the increasing use of *RTW's to do quick business trips with several world destinations. Being corps, time is money and time is valuable, so they were wanting to do it in the quickest time possible. Many of the 3 minimum stopovers were becoming overnight only without 24 hour stays. (True also in some cases for mileage runners, although these aren't considered much of a problem). In keeping with the time honoured tradition of gouging Corporate travellers as much as possible, by enforcing the 24 hour stopover the Corps were faced with either lengthening their journey, with the consequent penalties to their valuable time, or get a normal mileage or excursion fare that allowed minimal time to be spent in cities. Of course mileage fares are a tad more in cost than *RTW's and are what the carriers live on.
There were also issues with interpretation of the rule by carriers, agents and customers themselves for ticketing and re-issuing, so it was clarified to a uniform standard by enforcing the IATA general rule on stopovers, allowing no confusion.
Cheers.