The key to understanding the PFF opinion on running backs and the importance of the running back to team success is the expected points added stat. If you study this data it basically says the running back makes little difference. Basically, expected points takes a ton of historical NFL data regarding results of a play in specific situations--field position, time remaining, score differential, etc--to determine the value of a play in terms of points. For example, a four yard run on third down has a higher expected points than a four yard run on first down. Then you take the difference between the expected points at the end of the play and the beginning of the play to determine the value of that specific play.