Yeah, I think if that if after all that build up, all they used the ghost army for was to scare a few pirates away then it wouldn't have played very well on film. Again, this is the difference between print and film. In print, big action scenes can be oh so boring, even if well written. So most authors worth their salt tend to skirt around such things to instead focus on the characters.
I like to think 'Dune' has a good example of this is. The two big conflicts that take place in the book--the fall of house Atreides & the climatic battle of Arrakeen--take place almost entierly off screen...page...whatever. Indeed, the most gripping "action" scene in the whole book was just a one-on-one knife fight between Paul and that Fremen the first night after they escaped.
Actually, the Hobbit has a good example too: battle of the five armies. Bilbo get knocked out about 10 seconds in and Galdalf describes it to him later...and I'm pretty sure if they did *that* on screen and not actually show the battle, then I think even Tolkien purists would be pissed.