Well as we all know, when Mangekyou Sharingan is obtained you can only use it so much before going blind, proven by Madara hence he took Izunas eyes, and as to why Sasuke took Itachis eyes, and so I've heard you can only obtain Eternal Mangekyou Sharingan from a brother/sister... so with the constant use of kamui from both obito and Kakashi, why are they not blind or even going blind?