[mrmr]

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Actually, that entire sequence is quite different in the book.

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The importance of them was that by helping to defeat Sauron's forces in the south, it enabled the large armies of men to come to reinforce Minas Tirith.

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Not to mention loyalty, debt paying, suffering consequences of one's own actions, etc., are recurring themes in the novels.

And it isn't as if these ideas were cobled in so the "ghost army" angle could be pulled off later on.