A couple examples if you will:
Matt Carpenter destroyed the Leadville record with training runs no longer than 30 miles. Leadville is a less technical course and Matt is obviously a strong mountain runner, but its telling that he didn't train for 100m in the "traditional" manner.
Greg Crowther, who admits that he himself suffers on trails, beat some of the worlds best ultra runners (guys with marathon prs in the 2:1X) at Sunmart 50M last year. He said they looked like they didn't look very comfortable on the terrain (and Sunmart is as runnable as any trail ultra). Of course they took a wrong turn for a bit as well.
But that is part of trail running, its not just all out, rhythm running like on the road. Fueling on the run is a major factor. Muscle damage is HUGE factor. I wouldn't be surprised if the body type of a Geb or Tergat just couldn't handle the pounding for that long.
Again, its all moot. We'll never see a fast marathoner in their prime attempt a trail ultra. And I have no problem with that. Who really cares?
The whole discussion arises because wannabes who are wasting their time on a message board instead of focusing on their own training need to feel better about their waste of a life by putting down someone else's accomplishment which they deem pathetic.
The irony is that running (track and elite road running)is considered a fringe sport in our country that the general populace thinks is a worthless endeavor. Now all of sudden tracksters/marathoners are giddy that there is someone they can take their frustrations out on.