I know most of you were not around back when it used to apply a miss chance equal to his slow on hit and active. And as you can tell from reading this it was entirely unreasonable as a passive. But Riot hating the RNG system mechanics removed the miss chance completely rather than keeping it on active.
However, would it not make sense to afflict the target with a 100% miss chance on active? Here are some reasons and there are many more but I will leave those for everyone else to post.
1. It would make his late game viability greater. (Blinding the carry for 3 seconds could be very useful)
2. It would fit the lore considering it says he uses the same poison as Teemo's blinding darts. (And by this logic it should apply a DoT rather than flat damage...but that is an entirely different discussion).
3. Look at that picture, STRAIGHT IN THE EYE! That implies at least a 50% miss chance since one entire eye is damaged. But to reduce RNG system dependence 100% blind is better.
Well think about it. I am not sure if anyone else believes Shaco's viability late game is just too subpar to merit not getting this buff.