"If you have a larger group that includes mages, you'll be throwing away any skill points spent on mage spells in return for some priest skill. You're also giving up all hope of casting Long Light, Major Blessing, Unlock, Fireball, Firestorm, Kill, and other useful spells."
fireball and firestorm are replacable with flamestrike and divine thud, but it would be hard to go without mainly long light and unlock. (lamps and piercing crystals would just be annoying to have to carry around in mass). If priest skill is maxed out for all characters, then you can go the cheap way and replace major blessing with avatar.
Kill and summons would be a big loss though.