What the hell is a cleric?
Originally, I pictured them as holy warriors - knights dedicated to a god. They wore heavy armor and they were pretty good at fighting.
Then the paladin came along. It pretty much eclipsed the role I'd imagined for the cleric. Clerics then got shoved into the role of religious leaders/priests.
Of course, priests usually stay in one place and tend to a congregation. They aren't adventurers. Could they be missionaries? That would handle the moving around, but there was never really any text to support such an interpretation.
Also, what's with the healing. Why are clerics all medics? Sure, there are some traditions of faith-healing and such, but the basis for this archetype in fantasy literature is very weak.
Paladins, as holy champions, make sense. Clerics, I suppose, could be holy champions who concentrate a bit more on prayer than combat than paladins... but that's an awfully fine distinction.
I could easily see, however, a class that replaced the cleric with a more clear-cut priest/missionary type. They might be healers, but they'd more often inspire their allies to heroism (fanaticism?) in the name of their god and the ideals he upholds. Charisma would probably be as important as wisdom (if not more so).
Such a character wouldn't be as easy to play as a traditional cleric... and in many groups it might not really be a socially-acceptable option. I think it is interesting, though...