Sexual selection may also come into play. Not all traits are strictly beneficial in the sense you are thinking of. Case in point: peacocks. Those big, beautiful tail feathers are great for attracting peahens, but horrible for avoiding predators.
Yeah, that's what i was thinking. Perhaps our mutated hairless predecessors were instantly more attractive to the opposite sex, and everyone wanted to mate with them.
As a member of a pretty hairless ethnic group, I second this emotion. I never saw really hairy people until I went to college, and boy what a shock.
Humans with hair.... on their backs? 
Since I am married to a hairy white person, I obviously got over my initial surprise.
But...How did
hair-covered lipless white people ever decide that smooth shiny black people were the ones somehow closer to
hair-covered lipless apes?
