Sight for sore eyes? First of all, why are your eyes sore? I've never had sore eyes. I would go see a doctor if I did. Second, what kind of sight is specific for sore eyes? Are the sights supposed to be restorative, to make the eyes less sore? Or are they sights that are somehow responsible for sore eyes? Whatever you can look at that makes your eyes sore, I want no part of it. Then again we are talking about humans, and their eyes are nowhere near as good as cat eyes (especially in the dark).
According to the humans at The Straight Dope, the saying takes some obscure definition of the word sore to heart. In this context, "sore" actually means "afraid". Of course, how obvious. Human words can't just have one meaning, they have to have five or six increasingly unknown meanings and uses. So instead of saying "I am glad to see that, it makes me less scared", they say "That is a sight for sore eyes".
Humans are just plain dumb. SAY WHAT YOU MEAN!