I always had the impression that while the Orcs where fairly proficient in metals, due to Sauron's influence, they had neither the ability nor the inclination to work farmland. So that might be why Sauron used slaves, also, not having the Orcs growing their own food allows more of them to be used for military purposes.
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"Outside of a Warg, a book is a man's best friend. Inside of a Warg, it's too dark to read." - Wargo Marx
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