Quote Originally Posted by BananaFob
Whether or not that Mangekyou Sharingan is takes a toll on the eyes, I doubt it will play a part in the storyline. If we had a Sharingan-less Itachi going around, Sasuke would kill him faster and won't prove much of a fight to us.
While SPLASH! made an excellent argument as to why MS damages your eyes, I think Banana here has the right idea: I seriously doubt it will play into the storyline significantly. If MS DOES pose a risk to your eyes (and it quite likely does) then Kakashi was only mentioning it as a way of foreshadowing it before he uses it in his fight against Deidara. In which case, everyone is right, in one way or another.