But then all they had to do was have Treebeard explain that the Ents are not hasty and that it is vital to rouse them.
Nonetheless, reading these posts has given me another idea. We know how PJ loves his twists (see
'The Lembas Saga and the Return of Sam') so it could well be that he just wanted the kids in the audience to feel sad, then happy, by the Ents' decisions.
I don't like this. I think that twists are overused; and that there's nothing wrong with showing Treebeard itching to go to war from near the start of the film.
EDIT:
Boromir already made that point four minutes ago, it seems.