I can see Arwen's point: if one is the impatient type, then characters like Treebeard, Merry, and Pippin do slow the active parts of the film down. But they also show that real power does not reside in the flashy action of the surface: it's far more awesome when it comes from the depths and shows itself as a last resort. I think breddelwyn hit the nail on the head when she credits Tolkien's love for the natural world as the source of the character. Treebeard does, indeed, remind us of the power that resides, most unobtrusively, in the natural world. Like Saruman, we ignore it at our peril.