I dunno Doug, but you can talk about planning in pretty abstract terms that have paractical implication.
As far as knowledge is concerned - no perfect knowledge exist, to be sure. However, planning can be much closer to "perfect knowledge" than pure market, because it can take advantage of the economies of scale in reducing "transaction costs." (see Oliver Williamson, _markets and hierarchies_). The argument here is that obtaining information costs money (transaction costs) and individual actors operating in a "pure" market environment might be overwhlemed by that cost. "Hierarchies" in Williamson's terminology (i.e. administrative planning) reduce (but not eliminate) that cost in two ways - first they reduce the amount of transactions among independent parties that require "knowledge" to be effectively monitored, and second they pull together (i.e. socialize) resources thus making obtaing information more affordable. From that perspective, corporationa can be concpetualized as middle-range planning that diffren from centrally planned economis only in the geographical scope of planning. Central planning usually corresponds to the geographucal boundaries of nation-states, whereas corporations are not bound by geography. (there is another issue of monopoly or quasi monoploy or niche which is another story).
So the issue of planning can be conceptualized and defended on a highly abstarct level - and more importantly, even within the essentally neo-classcial conceptual framework (e.g. transaction cost approach). The real problem lies not it its "theoretical efficiency" - because it can be theoretically defended - but in its working in real life.
As institutionalist critics of Williamson's work pointed out - formal-rational organization (i.e. "hierarchy" in Williamson's lingo) is but one form of social institution - other forms involve informal social networks, solidarity ties, etc. So the real question is how these two diffrent types of institutions, formal-rational (planning) and informal ("spontaneous" social networks, norms, expectations etc.) interact with one another. In some cases - the informal ties and norm can reinforce formal-rational institutions (this is argued, among other by Granovetter, Economic Action and Social Structure: The Problem of Embeddedness, _American Journal of Sociology_ 91(1985):481-510), but they do not have to.
In fact, central planning of Eastern Europe is a prime example of a conflict between formal-rational institution of central planning, and informal instiutions or "shadow economy" -- see my piece on that titled "Beneath of Veil of Market Rationality: Cognitive Lumping and Splitting in Narratives of Economic Development" in Kinloch and Mohan (eds) _Ideology and the Social Sciences_, forthcoming (an earlier version can be found in _International Journal of Conemporary Sociology_ 35(2), 1985, 172-188). In that piece I argue that the multiplicity of diffrent types of instituions: central planning, market mechanisms gradually introduced since 1956, and informal solidarity ties that coexisted together and often contradicted each other - is the factor that increased rather than saved on transaction costs of central planning in eastern europe.
So the bottom line is thatthe goal of planning is not to devise a perfect master plan, but to coordinate different types of social institutions both formal (public policy, economic planning already practiced by governments and corporations) and informal (social solidarity networks, "social proximity organizations" like churches, cultural heritage or ethnic groups etc. interest groups, etc.) reinforcing each other in the promotion of "desirable" behavior. For those informal institutions can either derail or reinforce the formal-rational ones - so th eidea is to make the two cooperate rather than contradict each other. This, I belive is also the marxist idea of unifying theory and social practice - "theory" in this case formal-rational institution of planning being informed by "social practice" or informal social institutions of "common folks." This is an open-ended process rather than designing a specific policies.
I also guess that is the bottom line of the recent brouhaha about "civil society" which is thought to capture energies of informal social instiutions and align them with formal-rational ones (see for example Robert Putnam, _Making Democracy Work_).
wojtek