I find it quite fortunate, that they don't use it automatically. Introducing a 1e-3 relative error is quite a deal breaker for some. Not for games sure, but for science that is mostly unacceptable.
From memory, GCC does one NewtonRaphson iteration on the approximate result so the error is much lower (closer to e-9 from memory again). They don't use the approximation directly in fast-math mode.