[plaidphantom]

A harmful attribute on its own can be advantageous when combined with others. Think of it as finding a new "local maxima" in the sum total of evolutionary traits. Being (probably randomly) near-blind may have led to being slow and careful, which was the temperament needed to survive climbing trees, which got them away from ground-based predators and found them a lot of leaves ground sloths couldn't reach.

Evolution is basically a random-walk algorithm to find combinations that work.