There really isn't any reason to not vassalize him. He can't call Crusades or the like, and I believe the only "invasions" he could grant would be against other Miaphysites. Which, if you're Nubia or Abyssinia you've probably already conquered the other and Armenia has long since fallen, and if you're Armenia odds are Nubia and Abyssinia have long since fallen. Not only that, but having him as your vassal should give you even greater range to excommunicate people.
So no, beyond maybe RP reasons there isn't any benefit to not vassalizing him. Then again, RP reasons are always valid reasons if the goal is simply to have fun and not try to be as disgusting of a blob as possible.