Preparing a running back to handle the coordination of pulling a defender while they are trying to tackle him and improving speed are two different concepts.
Vern, as do many others, have these guidelines because once you have altered mechanics, you are going to negatively effect the runner’s efficiency. While a defender pulling against the running back has merit, it is situational. What if the 100m had a 8m headwind? What if it is raining? What if there is a cross-wind? What if, what if . . . by all means, you want to practice these things from time to time, but the important thing is hitting the hole as fast as possible. Which, dragging a heavy all the time is going alter mechanics and wire the neuromuscular system to run slow and heavy. You want to optimize speed and quickness while occasionally working on other aspects. I would rather have the running back that doesn’t even give the guy a chance to grab the jersey or work on ways in which to avoid tackles.
I wouldn’t advocate pulling heavy sleds all the time for anyone. But what things, other than contact time, are altered when pulling a 20%bw sled vs a 10%bw sled? Keep in mind I’m talking about the first 2-3 explosive steps which are critical in many sports. Also, I remember some of the hills in CF’s GPP that looked steep enough that they would slow the athlete more than 10%.