I agree that the object is to run the marathon, you throw in a little bit of walking if absolutely have to, but you miss the point of the accomplishment if you set out with the intent to walk. After all, the marathon is a running event; there are events for walkers if that is what you want to do. If you are not ready to run the whole distance, you need to train a little longer and little harder--after all the marathon was not meant to be approached casually.
With that said, marathoning as a business would not be what it is today w/o those 4 hour runners. It takes a lot of money to host a world class marathon and those 4 hour runners foot the bill. Btw - a 4 hour marathon is not walking and it is a significant achievement for someone who is not a runner. I've run the 800m faster than probably 99% of the people on Letsrun, later in life I took up road racing and my first marathon was only 3:38. I trained hard for that marathon and ran to point of muscle spasms, in fact, I had to walk twice. --I hope no one demeans my effort, because I have since run much faster marathon, but that 3:38 remains my most difficult one and the one I am most proud of.