PS basically a MS technique, EMS is still MS but with the protection from blindess because over use of chakra to the eyes or strain to the eyes.
In indra case, we can assume that he is in special case as the first persons to wield the Sharigan that direct power born from Hago power.
but his descandents dont have that luxury to gain PS by MS alone, other than forbidden tech like Izanagi and Izanami, like I state above, using to much the eyes power can causing blindness
because of that Sasuke and Marara need a EMS to achive and stabelize PS, PS hold much power compared to not to refine susanoo.
In Sasuke case, I think Kishi want to drag him to use a PS in IT chapter, but He having shown the PS triait earlier when his Susanoo wrapped a Kyuubi.
In madara case, sure we knew already, it same as Sasuke cases
and both having EMS to avoided blindness, Itachi lost his other sharigan after activating the normal susanoo is the proof that power put some strain to his eye after using other MS abilities
in Kakashi case, Obito is not a living being but a Chakra and the rule of life is out for him, that mean kakashi not needed to fear blindness if that happen , and if that happen, his real eyes will not. But for me ad Obito is spirit with chakra and not following real life rule, blindness will not occur
and he can test the susanoo to the limit by copying Sasuke PS if he really using copying method, if not that can be consider his form of PS
That my two cents