The problem with RBs is that even the superstars are one leg injury away from being just another guy. Look at Ki-Jana Carter, Robert Edwards... the list stretches on. Even though he's all kinds of exciting, I get the feeling Adrian Peterson is going to suffer a nasty tear or something (worse than the current one) and regress mightily within the next year or two, as well, just from looking at the way he runs.
The difference between a great RB, a good RB, and an average one aren't all that great. Would having LDT (great) over Corey Dillon (good) in 2004 made that much of a difference? How about Dillon over Antowain Smith in 2001 and 2003?