to sum it all up:
kakashi cant use the sharingan for an extended amount of time because he is not an uchiha and it drains his chakra much faster. sasuke and itachi can maintain the sharingan because they are uchihas.
it was pretty much clearly stated(clear to me) in the manga that the MS deteriorates the eye site of the user, like adonai said why would they lie?
obito was just a cry baby who used the excuse "i have dust in my eyes"
"Our deepest fear is not that we are inadequate.
Our deepest fear is that we are powerful beyond measure.
It is our light, not our darkness that most frightens us.
We ask ourselves, Who am I to be brilliant,
gorgeous, talented, fabulous?"
Actually, who are you not to be?