Further down the comments, Monbiot is pulled up on this and says
'I agree with you on Marx's erudition and insight, but to me the Manifesto contains in theoretical form many of the horrors later visited upon the people of the Soviet Union and some other communist nations. Dialectical materialism reduces humanity's complex social and political relations to a simple conflict between the “bourgeoisie” and the “proletariat”; ie the owners of property and the workers, by which Marx and Engels meant, basically, factory workers. Any class which didn't tick one of these boxes was either, like the peasants, shopkeepers, artisans and aristocrats, destined to “decay and finally disappear in the face of modern industry”, or, like the unemployed, was “social scum, that passively rotting mass thrown off by the lowest layers of old society”, with no legitimate existence in a post-revolutionary world. But the world didn't work like that, and the people who didn't fit had to be shoved under the wheel of history.'
http://www.guardian.co.uk/discussion/comment-permalink/15002362