I'll type this slow so you can be sure to understand it.
Research has been done that clearly shows that you get very little more physical training benefit from a 100% effort race as opposed to a 90% effort workout. Yet that same research shows that the recovery period needed from a 100% effort race is twice as long as a from a 90% effort workout.
Lets say it takes these kids 1 easy day to recover from a workout but 2 days to recover from a race before they are ready for another workout.
Then by running only a race every other weekend as opposed to every weekend during a cross country season they are getting in 6-8 additional workouts in. That can make a real difference in fitness. Multiply this by 3 season a year and you have the additional fitness from 20+ additional workouts per year under your belt by having a more modest racing schedule. Multiply this by 4 years of school and that makes a world of difference.
This doesn't even factor in the additional travel time, disruption of training routine and stress that racing every week causes.
It pretty clear which is a better way to go about it.
As for Bill Rodgers, ask the man yourself as I have done first hand. 90% of those races he did were nothing more than good 90% effort tempo runs for him, he just happened to get paid for them.
But Lutz last 6 weeks before Foot Locker National were all 100% efforts for him and he looked like it in the last mile at Foot Locker.