And it's not true that resource can't be expended to achieve this. The cleric PC can spend resources healing the other PC. Or rebuilding his/her ruined house. Or could risk his/her life in combat to help the other PC, therefore illustrating the lengths to which the god in question will go to help the deserving. Even in the absence of a PC vs PC social conflict resoution mechanic, there are many ways to spend resources to persuade another PC to your point of view.
Yes, but none of these
make the other PC convert. Said PC could just as readily look at all these boons and declare "nah, I don't think I'll bother".
I don't understand this either - why is RPing that interacts with the GM's setting not its own reward also?
Because the DM can police it, to make sure that the players haven't just arranged to stage this dramatic conversion for the cheap XP. And because they game does include mechanics for influencing the attitudes and thoughts of NPCs - be that diplomacy checks, Skill Challenges, or whatever. (The DM may choose not to use these mechanics, but in general if he does so it's because he's using freeform roleplay to the same end.)
Still, it's a good point - I'm not really a fan of XP awards for roleplaying at all. (Or, indeed, "quest XP" either, although less so.) These days, I just give a fixed number of XP per session.
If it took the form of "Write my god's name on your character sheet, then I can heal you without having to atone" of course no XP are required.
That's an interesting argument for a player to make - what with their being no such requirement to atone for healing characters of other faiths.
But if it actually played out in character, with arguments being put, offers being made, etc then it strikes me as entirely XP worthy (under a general RP/Quest/etc XP framework).
Perhaps.
I'll note again something that I think is pretty key: between this thread and the wish thread, it appears that the OP is a novice DM with more experienced players, at least one of whom seems to be trying to take advantage of his good nature. That being the case, while I would have no problem with an experience DM doing otherwise, my advice
to him is not to allow it. Because to me, this feels like another attempt at abusing his good nature. I may be wrong about that, of course.