A snip (with emphasis added by me) from Will's splendid column on Castro:

Communism of any stripe is afflicted by terminal ignorance. Having no market, which is an information-generating mechanism, communism cannot know what things should cost.

Hence communism's amazing contribution to humanity's economic history is "value-subtraction" -- products worth less than the materials that go into them. That result is seriously inconvenient for Marxism's labor theory of value -- the theory that labor adds all value to the world's materials.