After all the posts about what should XP be awarded for, how it should be used to spur gameplay that matches what the DM and players want out of the game, and how at least D&D is set up to reward certain actions, I've come to this conclusion.
Let's start with our fictional missionary Cleric. He's gone on his share of dungeon crawls and wilderness adventures. He's faced some monsters, recovered some gold, and found a couple nice magic items in the process. Maybe he's even made a bit of difference in the Grand Scheme of Things toward his church/deity's goals.
Now he wants to spend time converting the masses to his way of thinking. He stops going on so many 'adventures' and focuses his time on building his congregation.
Should he receive XP for this? I'd say in general, no. If the player is having fun running the character that way, then by all means continue to do that. And the character will reap in-game benefits such as wealth, power, prestige, and followers. Those are the rewards for that sort of activity.
Now, I can see awarding XP for this if it becomes something the ENTIRE PARTY decides to devote their time to. If the Cleric is out there preaching it, while the Fighters are protecting the flock, the Thieves are rooting out spies from other sects, and the Magic-Users are researching how to take down the avatar of the Chaos Frog God of the Swamps, then I can see awarding XP for it.
Otherwise, it's doubly rewarding divisive play. It's great if everyone has personal goals for their character. Maybe one Fighter in the group is recruiting a crew to go viking with him, while another is training to win the King's Joust and be knighted. The Thief is working on building a network of contacts to create a rival Thieves' Guild. The Elf is creating intentionally cursed armor and trying to come up with a scheme to get it to an enemy as a "gift." The Halfling is searching for the perfect seeds to grow the best crop of pipeweed ever next season.
But all of this should happen in the background, in solo sessions, or just as color. Otherwise, everyone's sitting around the table bored while one character is being played, or everyone's fighting for the DM's ear to get their shot.
Actually, I can think of one instance where I would reward that sort of play where it's not the focus of the party. It's if we're using the Arneson rules of spending your hard earned gold to convert it into XP. If you spend it on that proselytizing then you get XP for it.
By all means roleplay it out. That's, I think, the fun of the Arneson carousing rules. Getting to roleplay out how you spend that gold to get XP adds lots of depth to the characters (or at least it can).
Then, I think, we could have the best of both worlds--adventurers focus on plundering ancient tombs and dragon hoards, then focus on their own personal goals once they've gotten those treasures out of the dungeons.