The bats did as much in the movie as they did in the book. Actually more if you count what Legolas did, which I liked. I think within what Jackson is doing they're thr 5th army.
Well PJ does not seem to mind "improving" other parts of the Tolkien's story... So why not the Bats... Personally I did not like the Legolas bat ride. Just more silly "Look what Legolas can do" when they could be spending more time with the actual story... Or making the Bats cooler
I thought the five Armies were
Dwarves, Humans, Elf's Orc's and Eagles... Bats I think are considered part of the Orc army as they are not thinking beasts like the other armies. Eagles fit more of the Army definition then the Bats do... Hell perhaps he does not count Bats or Eagles... Perhaps he was counting the Goblin's that showed up for 3 seconds...
But I think it's the Eagles without question.
What were the five in the book?