User blog comment:Argonash11/Why can't Quarians have see-through visors?/@comment-260745-20100713132440

I'm pretty sure it was intended to give the Quarians an air of mystery (you will recall that in the Codex, it says that few non-Quarians have seen what they look like under their suits), but the reasoning within the Mass Effect universe is probably that the the darkened visors protect the Quarians' eyes and skin from UV damage.