I think that in general we have a pathological response to anything we measure. We tend not to measure the thing we care about; we tend to measure something that indicates its presence. It’s often very hard to measure the thing that you’re hoping for. You don’t actually care about how calories you eat; you care about how much weight you’re going to gain from the calories you eat. But as soon as we go, oh, well, calories are a pretty good proxy for weight gain, we start to come up with these foods that are incredibly unhealthy but nevertheless have very few calories in them. In the same way, Google doesn’t really care about inbound links because inbound links are good per se; Google cares about inbound links because inbound links are a good proxy for “someone likes this page; someone thinks this page is a useful place to be, is a good place to be.” But as soon as Google starts counting that, people start finding ways to make links that don’t actually serve as a proxy for that conclusion at all.
I can think of another example of this phenomenon.