If you wanted to watch the best of the world compete you could watch the Olympics. Usually it like the Kenyan olympic trials anyway so why not just make it the US's best instead.
I want to see an American win Boston again. If a weak field is the only way this is accomplished that's fine with me. I'd rather watch these American's duke it out than see a bunch of East African's race each other. No offense to the East African's, but you can see that at 100+ marathons around the world every year.
I also think any chance you can get the best Americans together in a race, on a great course, with good crowds and decent prize money you should take it. Races like this help strengthen US marathoning in the future.