Just spitballing here a little off of Som's musings. I have no doubt that Meppa was kept alive by Kellhus for a purpose. I've been thinking all evening what that might be.
1. Just for giggles (and hope that my Moe=Meppa might still come true), what if Kellhus recognized Moe had a hand in Meppa?
2. Kept him alive for studying the Psûkhe, and what about it that leaves no Mark.
3.He has another head to attach to him, a là Malowebi.
4.Try and make Meppa see the light, that they are indeed on the same side and persuade him to join him in the fight against the Consult and 100.
5. Som's take, that he was an instrument of Kellhus all along and was whelmed. Will reveal to him a là Proyas, he was instrument being used, and still has a purpose. Hell, he'll probably even bugger him just to get the point across. Seemed to work with Proyas.
My guess would be #4. That Meppa is such a potent weapon and Kellhus feels he can convince him to take his fight to the Consult. We know that Meppa realizes what the 100 are, and I assume he has deduced their role in damnation. I agree, that his story isn't done yet and will play some role in TUC.