Why the Sharingan?
"So all this is about that eye? You disappoint me, my student. Perhaps, when I called you my most gifted pupil, I was wrong and you were, in fact, an idiot."
"Kukuku... Why would you say such... amusing things, sensei?"
"Think Orochimaru! The Sharingan teaches it's user by copying others. Thus, it will only copy up to the level of whoever you are looking at! You have to master the copied techniques yourself! And what's worse, if you later copy someone else they will get rewritten!"
"...what?"
"It's an eye. It can't think. It can't choose. It can only copy. See a punch and you can do the same punch up to the limits of your body. So what happens if you just copied a perfect punch and then glance at an Academy student? How can an eye tell which punch is better?"
"..."
"What do you think happens to the old punch?" Sarutobi was practically shouting at this point, filled with passion as he tried to reach his student one last time.
"No. No. If it copies everything then you should have both punches in your muscle memory, you could simply use the better one."
"In the heat of combat you would choose to use which of however many sets of reflexes you have? Even if you keep all the reflexes you copy you would choose the most current one, the one your opponent is using. Fight a Jonin and you fight like a Jonin. But fight a Genin and things change. It's why Kakashi keeps his eye covered except against the strongest of enemies. Besides, if the Sharingan was so awesome don't you think every Jonin of Konoha would be sporting one after the Massacre?"
A/N: Actually, it wouldn't surprise me if the Sharingan can magically tell if what it is copying is better than what the user already knows. It can apparently do anything else after all.