Thinking about it, maybe PJ did a better job of character development than Tolkien did. In the book the whole work with Boromir is to show his allegiance to Gondor and his warrior status in Rivendell, then he is a strong brave man on the trek, and finally when they (spoiler alert)
start down the Anduin from Lorien he starts obsessing about the ring, paddling up close to Frodo's boat, mumbling.PJ uses the scene with the hobbits to wrap everything up and show that the ring is going to succumb. I would have liked to see it take a little longer, but then we get back to the seven hour movie that makes a dollar forty-seven cents.