There was no point of keeping it. Obito might give him the left Rinnegan. Anyway there was some thread that predicted that Madara will have Kamui with Obito's right eye but it turns out it was actually going to be Kakashi.
After that, he will most likely stomp the left eye. That sharingan is almost as worthy as Shisui's. I wonder what Shisui left eye does since the right eye controls minds.I don't think Madara intends to keep Kakashi's/Obito's left eye. He just needed to get to the Kamui dimension to collect his other Rinnegan is all.
Both of Shisui's eye had the same power. Just like obito's.I wonder what Shisui left eye does since the right eye controls minds.