I think it's also important to look at how successful teams don't necessarily have one workhorse running back who gets run into the ground all year long. Take a look at the teams who have recently had deep playoff runs...
-- Giants in 2007, leading rusher was Brandon Jacobs with 1009 yards
-- Arizona this year, leading rushers were Hightower and Edge -- combined for just 913 yards
-- Pittsburgh this year, Willie Parker gains just 791 yards
Also, check out the stats over at NFL.com for the
running backs who had the most carries. Look at how many of those teams made it to the playoffs. You have to go down to the guy who had the 12th most carries (Chris Johnson of Tennessee) to find out who had the deepest playoff run. At #17, 18, and 19 it was Brian Westbrook, LaRon McClain of the Ravens, and Brandon Jacobs. With the exception of Adrian Peterson of Minnesota, every other team who had running backs that carried the ball more than anyone else either didn't make the playoffs or were 'one and done' when they got there.
So I think this shows that we have placed too great a priority (and spent way too much money) on the "Superman" theory of running backs. You know, just hand the ball off to one guy and he will singlehandedly carry your team to the promised land. All that does is burn him out of gas, and it makes your offense dependent on the health of one guy.