My guess is that the mutations are retained as variation in the population because having some small subset of them probably results in divergent thinking which can increase overall group survival. Autistic spectrum disorder individuals result from having too many of these mutations, such that neural activity patterns find a stable equilibrium outside of "normal" (think strange attractors in system dynamics).

Evolution isn't directed.

Either genes survive or they don't. There's no forwards or backwards.

dragonwriter is right in general sense. We can evaluate it, though, from our own perspective. It's a step forward in terms of intelligence and stability in world. It's a giant leap backward in terms of success or happiness in a world of non-autistic people. Evolution would also consider it a negative in that autistics get laid and reproduce less than non-autistics. They also are more likely to loose in competition in corporations, battlefields, and jungles. So, it's a net loss overall for evolution's main function but autistics persist for one reason or another.