@XanBcoo: Because deteriorating eyesight is something only a Mangekyou user must experience. If every Sharingan user faced that problem they would be a really pathetic clan, as they would all be blind after using the Sharingan a few times!

Anyway, its never been mentioned before that normal Sharingan users have their eyes damaged which means it must be something unique to a Mangekyou user. And I doubt Kakashi just heard it from someone or read a book, as there have only been like 3 users of the Mangekyou Sharingan. Information must certainly be scarce on it.

Therefore, the only way he could possible know is if has experienced the problem himself. The ONLY way he could know this without having the Mangekyou Sharingan is if a Mangekyou user told him, which I doubt.