Having just had the opportunity, I made a prophet who is a level 6 priest, as well as having good regular magic (2 astral, water, air, I think).
Now, I'm very well aware of the huge benefits of obtaining a level 3 priest or level 4 priest when your kingdom doesn't natively produce such. Sermon of Courage, Divine Blessing, great joy.
Level 5 seems more iffey - 100 accuracy in a spell doesn't mean what I thought it meant, as my arch-theurg prophets call down bolts of lightning with incredible accuracy - on empty tracts of land. Smite just doesn't seem worth it, given that I've watched battles where 5 of 5 castings didn't hit anything. (Bug? Or do I just not understand accuracy?)
So - any benefit to having a level 6 priest? Holy spells have 0 endurance cost, so one major benefit of high magic level is lost - 1/3 of 0 is, well, 0. There's no 6th level priest spells, not that this is usually an issue.
But, having Bartholomeus the Patriach show up on turn 4 with Pythium, and me not having made my ArchThug, er, Theurg, prophet yet because I wanted him in the hall of fame by attacking an undead area.... Hard to resist.
Now I'm thinking maybe I should have promoted Mohammed (the presumptive Prophet) to have two level 5 priests instead.
Any thoughts?