and Bilbo going on an adventure because he's basically bored, doesn't feel very Joseph Campbell.
If that were the storyline that the movie went with, I would agree. However, it really isn't. While, just like the book, Bilbo seems to initially choose to go on a whim (I wouldn't quite say boredom, it looks more like a guilt trip), the movie expands his motivation show that he really identifies with the Dwarves quest (although, logically, if this is their quest, they don't need a burglar). Your criticism is indeed a valid one for the book, though, which never really does give him much motivation. As a children's book, I certainly never thought about it as a kid, though. The movie does give him a stronger motivation than that and hammers you with it so I'm surprised you missed it. They literally ask Bilbo at one point and he says specifically why he is helping them on their quest.