I know this is a long shot... but Sasuke used the MS right before he used the chidori blade to hit danzou and karin at the end of 480. However he doesnt need to use the MS to use chidori. so why use it (especially putting more risk on his eye sight).
We know kakashi can use ms to use space time jutsu and open up a portal to another dimension. We also know madara/tobi's space time jutsu can make all attacks go through him. What if the reason sasuke used MS was to use some sort of space time jutsu to make is attack go through karin.
As i said i know its a long shot, (would even be another super powerup for sasuke), but i don't see why he had to use his MS for his chidori blade.