Measuring job performance is hard. A good analogy is measuring intelligence. The IQ test is clearly bullshit (it does not come close to reflecting the complexity and high dimensionality of human intelligence). That said, it's a useful research tool. Across a large number of people in aggregate, it can be used to learn things (like that leaded gas was a really bad idea). We'll be doing the same sort of (very imperfect) measurement of job performance.