The sharingan is Sasuke's power, why isn't he allowed to use it? Using Amaterasu, Susano'o, Tsukuyomi, ordinary genjutsu, copying techniques, predicting movement and jutsu, all those are possible because of his kekkei genkai: sharingan. I don't see a problem why people complain him using something that's his power, especially against the 5 Kage where he had no choice but to use Susano'o and Amaterasu. The powers of the mangekyou come because of the desire for power over losing the person closest to you. I don't think Itachi really meant the Uchiha clan wanted to kill their loved ones simply to obtain power because if that was the case, why are there Uchiha still? No one seems to want to kill any of their loved ones. Because of the emotional trauma it causes losing the person closest to you, it gives birth to great power to protect others so that you don't lose anyone else, at least that's how the Rikudou Sennin would've wanted it. It's only strange that EMS is only acquired through taking someone else's eyes, which itself is a selfish act in a sense, unless people can acquire even eyes that have lost their light and still EMS. I don't know. Or maybe blindness is caused by the over use of that great power, more like the blindness isn't something that happens automatically, but is caused by the abuse of your power, who knows? It's still interesting to think about.