I feel there is no fair comparison that you can make between the priest classes and magic users. Each has his/her own specific areas of expirtise in which they shine... and compairing the overlapping areas is not very fair.
This game was meant to be a group adventuring game, so can't we all just get along? Proper division of labor in groups is essential to have a efficient party...
I myself know that a cleric/druid will always have his rear handed to him by a mage, but I would rather have the priest in a non-boss combat situation.