thats where your wrong ... kakashi and itachi use and abused the ms... it didnt drain the chakara its just everytime u use it you loose the light in your eyes... when saskue faught danzo saskue spam sasunoo alot more than he did at the kage summit(he didnt even use full sasunoo at kage summit) and he didnt use the flame shield once...I believe it was ms that cause sasuke to lose his chakra rapidly cuz he was in so much pain, now with ems he feels no pain. So far he used susanoo with no problem, he shot 3 susanoo arrows since he showed us ems an used amaterasu an genjutsu with no pain in his eyes.