I'm not saying SEO is not effective, it certainly works, but it shouldn't, and it is Google job to make it happen.
On a high level good sites don't benefit from SEO, because it wastes time and money on something that should be granted: good sites should be ranked high, crap sites should be ranked low. SEO is a cargo cult (http://en.wikipedia.org/wiki/Cargo_cult_science) with no benefits to society.
If SEO is provably effective, it is by definition not a cargo cult. The defining feature of a cargo cult is that it's questionable at best whether their practices lead to the goal they want to achieve (e.g. making fake walkie talkies and yelling into them hoping that a plane will drop cargo because you saw someone with a real walkie talkie make that happen).
On a high level good sites don't benefit from SEO, because it wastes time and money on something that should be granted: good sites should be ranked high, crap sites should be ranked low. SEO is a cargo cult (http://en.wikipedia.org/wiki/Cargo_cult_science) with no benefits to society.