I would argue that his approach was entirely reality-based, that was his major methodological contribution to economics. He argued that your position in society (your political consciousness) is dictated by what you have (your material conditions) which is the base-superstructure framework. He looked at society and saw two classes (with some exceptions) which were in conflict: the capitalist (who owned stuff) who oppressed the workers (who only had their labour to sell). You may not agree with what he saw, but I don't think the argument that he wasn't basing his work in reality is really valid. (I guess it would come down to whether you accept multiple subjective realities though, which is perhaps another debate.)
I'd have to completely and vehemently disagree that Marx and Keynes are similar. I find them worlds apart, besides the fact both are at times incomprehensibly wordy. As theorists I think they begin from very different places, and the way in which they develop their arguments is wholly different. In addition, Keynes had an entirely orthodox economics education, and I don't see any evidence of him adopting a single one of Marx's ideas. (If you want to see a Marxian-influenced take on the same things Keynes wrote about, try Michel Kalecki.)This said, the communist manifesto is better. I think the type suggested for him is ILE, and I agree with it. His writing style is very similar to Keynes'. It's interesting to compare the two authors because they are very similar, yet it seems that Keynes has been able to focus more on actually existing processes, but with less success as far as moving masses goes. The opposite can be said about Marx. I might say, that the latter has succeeded in fulfilling his hidden agenda, whereas the former payed tribute to his attempt at injecting into his system.
Marx tried to create a holistic and even hegemonic theoretic stucture in the model of communism, which I'd argue is evidence of Ti. Whereas, Keynes' approach, even if he tried to call it a 'general theory', is far more ad hoc and locally contingent, and evinces far more Te.
The labour theory of value, as written in Capital volume 1, is a simplified construct used to explain his circular flow model. I would argue it has as much utility as any other simplified model, like the 2-sector model used in mainstream macroeconomics. Marx did acknowledge that it was flawed, and was going to develop it in subsequent volumes of Capital. Unfortunately (or perhaps conveniently for him) he died before he could do so, and Engels was certainly not up to the task of finishing his work. I can't say any subsequent Marxist has either fixed this, and as you said, many have just accepted it. But I don't think their sin is any greater than any other economist who relies upon formalism and models with all their built-in assumptions.It can't really be said that Marx system will never work. He acnkowledged that what he wrote in his first books was not logically sound (his theory of value, as every labor-bases theory of value, is clearly wrong and from this follows the impossibility of determining prices). He was trying to solve this problem when he died. It's somewhat hilarious that some people tried to apply the ideology without first correcting this very major logical flaw.
I will apologise in advance for jumping into someone else's debate here, but it seems like you're applying neoclassical/orthodox reasoning to Marxist theory. Which strikes me as comparing things which have very different assumptions.
I don't think the communist model, when operational (so going on the hypothesis it can occur here), has any need for supply and demand (ie, the market) to distribute goods, or capital accumulation to incentivise because the whole point of communism is the collective - so that's everyone, not just the government or a central body, owns EVERYTHING. Thus there is no need to get one up on your neighbour and covet his property, as there is no sense of individual property ownership. This is based on a very different way of thinking to our present economic system.
Ezra, what you describe is a barter economy, which is not communism. Communism, according to Marx, can only occur after capitalism has flourished and then run its course, so it is definitely not about returning to a pre-capitalist mode of production which is bartering. It is also not about favours - in fact, the principle of self-interest still applies. Because everyone collectively owns everything, it is still in your interests to make your resources grow, but by doing so, it would be for the betterment of everyone, not just yourself. The reason why this collective ownership is necessary is to get rid of the alienation workers feel from what they produce. So the socialisation of the means of production means there is no disconnect between workers, production and consumption.
The world Marx was envisioning would be radically different from ours. So judging it by indicators we use for capitalism obviously does it no favours and just makes it appear ludicrous. Communism does not imply central planning, and Marx explicitly wrote that he did not think Russia would move to communism because, when he was writing, they just were not advanced enough in capitalism for the natural progression to be made.