Rashard Lewis Revisited
I feel like Dave Berri's missing my point here: "Now we have the argument that the value of Lewis should not depend upon position played. The numbers tell us that playing Lewis at power forward will cost Orlando rebounds. But we should ignore this fact and simply give Lewis extra credit for making an effort."
No. This is what I'm saying. Suppose you have two players. One is Rashard Lewis, excellent small forward (according to Berri's numbers) and average power forward (again, according to Berri's numbers). Now you have a second player. Call him "Lashard Rewis." Rewis puts up Lewis' exact same numbers, but if his coach tries to insert him as a power forward he refuses to play. Which player is better to sign -- Lewis or Rewis? Berri says it's Rewis -- Rewis will have a better position-adjusted Wins Produced number. I say -- and basic common sense says -- it's Lewis.
In any situation where Rewis could help the team win by playing small forward, Lewis can do it, too. But some situations will arise (suppose your starting power forward has fouled out and your backup power forward sucks, while your backup small forward is an above average player) where Lewis is a more useful player to have on your roster. It's true that teams employing Lewis do well to remember that he's much more effective as a small forward than as a power forward (assuming that's true) but it's also true that it's better -- more useful to your coach and GM -- to be able to "play out of position" with a modicum of success than to be totally useless.
That said, it's slightly absurd to even discuss positional matchups within the Wins Produced framework because it doesn't deal with defensive matchups at all. Is Player X quick enough to "downsize" and stay with his man? Is he tall and strong enough to "upsize" and not get pushed around? The Wins Produced framework doesn't differentiate between (very useful) players who can guard multiple positions, and (unfortunate) players who defend two positions because they're equally ineffective at both spots.