Fungible is an economics term. In common use it means: replaceable. A fry cook at Wendy's is fungible. However good she may become at anticipating the lunch rush, however inured she may be to splattering oil, and however evenly she might salt the fries, Janet the fry cook can never be irreplaceable. Joe can sub in and not burn down the building. For all her expertise, Janet's job just isn't that important.
Wendy's will never overpay Janet. She'd be lucky to squeeze out a ten cent raise without a corresponding cut to her hours. Janet may trace the delicate sinews of existence in her medium of bubbling oil and sizzling potato wedges, but she's never gonna get paid. Ever.
Fast food workers are a resource rather than a commodity. Alejandro might be a miracle worker with the sour cream gun, but Wendy's won't trade him to Taco Bell for a chalupa and an antacid to be named later. Excluding that technicality, the concept is the same: the harder something is to replace, the greater its value. Jill the sandwich artist might be as painstaking and expressive in her artistry as Pablo Picasso, but if Tuan can get it in the bag between bong rips, Fred the customer isn't likely to notice and Bill the boss isn't likely to care.
The most important part of a run game is success. A successful run game creates good down and distance, converts firsts, punches it in from the goal line and keeps the opposing defense honest. If a rusher could run for four yards on every attempt, he would be unstoppable. Four. Four. Four. All the way down the field. Like a machine built to destroy excitement.
Seahawks fans know something about the strategy of slow death. Coaches call it: bend but don't break. It's the strategy of forestalling loss at the expense of winning, and hoping, against all odds, that a turnover will occur before a score. I digress.
Our four-yard rusher doesn't exist. He can't create push or make his own holes. That's the job of an offensive line. The line wins the point and through a succession of small but decisive victories, allows the rusher to succeed. The rusher is fungible. A name attributed yards that are in fact earned by the entire offense. The sandwich artist of the grid iron. The underpaid Janet.
Or so is the prevailing wisdom.
Afraid it's more complicated than all that. Holes are transient. A slow enough rusher, like Shaun Alexander circa 2007, can lose yardage in a ravine. An agile and adaptive rusher, like Shaun Alexander circa 2003, can create holes by cutting back, getting skinny, breaking tackles and spotting lanes. No line can assure a back continuous success. The most successful running back of 2009, Correll Buckhalter, still only succeeded on 58% of his runs. And Buckhalter, for all his success, still wasn't very valuable. He ranked 18th in DVOA and 27th in DYAR. He could get you there, but that's about it.
C.J. Spiller stands head and shoulders above every other back in his class. He has great quickness to the hole, and so has better holes to hit. He has elite speed exiting the hole, and so maximizes the holes he gets. He's a special receiver and returner and the combined value of those skills can be as a much or more than that of his rushing.
Much is made about Spiller not being a feature back and therefore not being worth a top ten selection. Maurice Jones-Drew wasn't a feature back his rookie season, but he had more combined rushing and receiving DYAR than Terrell Owens, Larry Fitzgerald and T.J. Houshmandzadeh. And he didn't need Carson Palmer, Kurt Warner or Tony Romo passing to him to achieve that. Even Matt Hasselbeck can throw a swing pass.
The very concept of "feature back" is foolish. It's the same thinking that argues Ichiro cannot be a great baseball player because he doesn't hit home runs. Production is production and Spiller produces. The pound it out, four. four. four. rusher of yore is dying out. But the great running back is not. It's adapting. And smart teams are adapting their definition accordingly.
Not a feature back.
Not big enough to carry the load.
Just a running back anyway.
But the best skill position player in the draft.
And not even close to fungible.