I enjoy any thoughtful soccer analysis, so I enjoyed the article, but it also got me a little fired up. I'm not a big SPI fan and I wasn't too impressed with the accuracy of Nate Silver's World Cup article. I did, however, enjoy reading it.
My two biggest problems with what he does is what I believe is an over reliance on friendly results and an over reliance on player's league performance.
Regarding friendly results, I'm glad he is trying to weigh games with a relevancy factor, but unfortunately the relevance of most international friendlies is almost zero. For one thing, I think the home field advantage for friendlies is probably four times what it is for other games. Also, accounting for missing players is almost impossible. Extracting anything from a Rooney-less England game or a Donovon-less American game is virtually pointless.
As for rating players on league performance and translating it into international performance, the problems are pretty obvious. Again, I applaud him for attempting the breakdown and I enjoy the analysis, but it's all pretty pointless. This is an obscure example, but it's a really good one just off the top of my head. Asamoah Gyan of Ghana/Rennes(France) is a decent striker. He doesn't get much support in the French league and puts up stats that are a little above average. Nothing that's going to register on SPI both because of the numbers and the quality of the French league. But, the guy can put the ball in the net big time. This summer with Essien, Appiah and Muntari feeding him from midfield he's probably going to be one of the top ten goal scorers. Ghana's expectations this summer don't depend on whether he plays in France or in England, but their SPI does. Ghana's SPI would be higher if he played at Arsenal, but whether he plays for Rennes or Arsenal doesn't matter, he'll be very effective for Ghana this Summer.
Sorry if this was all kind of rambling. I really do appreciate the effort and the methodology Silver employs. I just don't think it captures the essence of the game very well. Ninety percent of a players statistical performance depends on who the other ten guys on his team are. You'll never be able to put that in a computer. Joe Mauer would pretty much have the same batting average regardless where he played in the American League, but Wayne Rooney's number would fluctuate greatly depending on who he spent a Premier League season with. His talent level wouldn't change, but his stats sure would.