MS has been said to eventually corrode the light in an Uchiha's eye with overuse. We have seen this in Madara, Itachi, and Sasuke. Now, we have seen exceptions in Tobi and Kakashi. I believe there are 2 significant reasons as to why this is.
#1 Tobi: Due to him having the 1st Hokage's DNA implanted in his body, it is giving him vitality and his eye won't seal shut like that of normal Uchiha.

#2 Kakashi: His eye was transplanted. He is not an Uchiha and therefore the only drawback for him is massive chakra loss. He has used the tech quite a bit over time and still has shown no signs of vision deterioration. In fact, with him transporting small objects, i.e, nails, he still has an amazing eye sight. I believe the MS drawback of blindess only occurs in Uchiha, while other shinobi will just lose a lot of chakra.

Thoughts?