he didnt use it against tobi because he was saving it for naruto ( to bring sasuke to his senses ) .even so he can't use it just like that, he isn't emplanted with the eye, he has it though, i doubt he could simply use it just like that, otherwise he'd probably have used it on Tobi.
if tobi would have attacked konoha maybe he did .
if itachi is in his prime in this fight ... he has totsura blade and shisui eye ... that is what prime means ... the best he ever had !