Like most posters here, I populate churches in my worlds with clerics and non-clerics alike. The only time this is a problem is when I use published materials, many of which seem to think that even the guy cutting the hedges outside the temple must have at least one level of cleric. As some Ars Magica book points out, in the medieval European Catholic church, the ones wielding power were not the best theologans, but the experts the canon law. The historic church is not the best example for D&D, as it was more powerful that one one D&D religion, and didn't have some members with magical powers. IMHO, a D&D religion would prefer to have Experts running the temples and such, as Clerics would be to valuble as healers, field agents etc. Besides, this gives us DMs an excuse for withholding goodies from players. "Well if it was up to me I would heal you guys for free, but the head of my order in this region has let it be known that free healing is only for the truly needy, and I know you adventurers usually have lots of wealth."