One thing I just thought of, is that if there weren't NPC priests taking up available roles, PCs could enjoy being a highly sought after Cleric of the Morninglord. Rather like taking potions out of the game it could be like handing over a role, that can be filled by a player, instead of an inexhaustible robot.
There's plenty PC's CAN do, but, as proven in PW after PW you can't have them hold the Highest tippity top positions, because PC's are people, and people get bored and wander off, or RL makes them do so for other reasons, then you end up with no one in that slot, and a crowd of folks wondering, WTF can we do now?
It's sort of like the issue of a DM run faction when the DM vanishes for one reason or another. When the guy with the house keys, doesn't show up to let you in the house, it sort of cramps the play of the moment. hence, you want to keep upper level stuff, as general as possible, while allowing PC's to do all the middle management they want, on their own effort.
~Rex