So saith Paul Krugman, following a long line of thinkers who believed in "economic harmony", "the invisible hand", and "spontaneous order", going back to the early 18th century. The fact is, the term is vacuous and highly misleading. A market economy exists only in the fantasies of those like Krugman. Markets are extremely unstable and have long been abandoned. Organization into firms is one very visible sign of this, as is the other elephant in the living room, massive state intervention in the economy.
The idiot "Nobel" prize economist James Buchanan (sp?) claimed that British society was "de-politicized" when the market came along (this in an essay in *Palgrave*). He apparently forgot about the enclosure laws and the actions of the imperial state which helped Britain to colonize a good bulk of the globe.
I don't favor all things being done by centralized planning, de-centralized planning has been shown to work to a certain extent to organize production in capitalist societies. I happen to think this planning ought to be organized slightly differently, perhaps along the lines that Robin Hahnel has advocated.