But why on earth is the Moth Priest still living at Castle Volkihar? I'm playing an evil Dunmer who is only using the Volkihar to attain immortality, why wouldn't I butcher this creep the first chance I get?
Spoiler
This is long after I've killed Harkan.
I swear this game does NOT want you playing anything other than an altruist, even when you choose the path that has you palling around with thousand-year-old vampires plotting to blot out the sun..
Well, as a good reason your character would want him around, he is your servant. He could be an EXTREMELY valuable asset for information. If you sent him back to the Imperial City (To the White-Gold Tower) you'd have someone very valuable on the inside of the city to leak information to you or to lure a moth priest to you that isn't blind so you can make another moth priest thrall and obtain more Elder Scrolls to be read so you can divine a possible future. You are immortal after all. Future events, no matter how far ahead in the future they are, would be of great interest to you.