Don't get me wrong Worlds are a great place to win if your a 2:07-2:08 runner because $60,000 dollars in prize money and appeareance fees is about how much money you would get at any given race(Unless you are an American, in which case Worlds are really not good at all because you won't win even if you are good and if your that good in the U.S. and are American you probably make loads of prize money). And for people trying to get back into the racing style or get their name out, Worlds are perfectly good for Marathoning, probably one of the best if not the best for that particular purpose as if you win Worlds people will probably start offering you good appearance fees and a win at Worlds could get you an Olympic spot. Also, the World Championships are pretty good to help pick up some WMM(World Marathon Majors)points as honestly getting into the top five at Worlds is probably easier than any of the Five big races(Berlin, Chicago, New York, Boston, London) and obviously much much easier than the Olympic Marathon which is obviously the hardest to get into the top 5 other than possibly the London Marathon its self.
The problem comes when you try to get really good runners like Patrick Makau, Sammy Wanjiru, Geoffrey Mutai, or Haile Gebrsalassie. The World Championship Marathon is almost absolutely no use to them as they already get loads of appearance fees and it would interfer hugely with their prize money, racing plans, training, and possibly World Marathon Major(WMM) points. So it's not going to attract the best unlike the Big 5 and Olympic Marathon.