I mean yes, people with disabilities often have a unique contribution to offer and a unique perspective. They often excel at very specific things. However, implying that it is so advantageous to be disabled that we would all evolve to be that way is ridiculous. Thinking things through slowly step by step may reveal errors others have glossed over...But it's perfectly possible for someone without a severe disability to do that. Meanwhile, the person with minimal to no cerebellum is totally incapable of stepping up to the task when, like a lot of the time, speed and intuition is important. If I wasn't able to rely on context and intuition at all I wouldn't be able to do my job, or even day to day tasks, with enough efficiency to get anything done. Furthermore, someone without a cerebellum will also have to learn to not smash themselves in the face when they're trying to eat and have to deal with constant sensory processing issues. This is not advantageous. It's a disability. So it's not going to be picked up by natural selection.