I'm sure they're looking for ways to make players stay around, and the best way for players to feel like it's THEIR game is to let them have a shot at adding content.
As you pointed out, they'll have to moderate things to install to prevent abuse of handing out rewards far exceeding the difficulty of the mission and avoiding cussing or anything along those lines. The whole thing will cost money to put up and maintain, so I'm a little less cynical I think. I think the most selfish thing I can imagine them thinking is that it's a good way to try out new writers without having to go through interviews.
I'm really betting the content will be very like the Elder Scrolls games like Oblivion. Some really awesome stuff, a lot of mediocre, and enough 'why bother?
and WTF?' to go around the earth a few times at the equator. Still, if they get in some good stuff that the players like, it'll help the game AND hopefully their paid writers will have a better idea of what players are looking for and later expansions will only get better.
Or it'll tank because they don't get enough good entries. It's an interesting idea though.