Is the fundamental theorem of natural selection of any use?

There have been many recent discussions of the Fundamental Theorem of Natural Selection, with an emphasis on its mathematical accuracy. It is argued here that, despite the mathematical problems that have been uncovered, it still has utility for biologists. In particular, it predicts an absence of additive genetic variance for fitness for populations at equilibrium under selection alone, a result that is valid under very general conditions. This raises the question as to why there are such high levels of additive variance in fitness and fitness components, but little evidence for non-additive variance.