Miles' brazen moral relativism (see below) not only makes it impossible to judge other societies, it makes it impossible to judge this society. If judgement implies a standard that transcends the given, which I think it does, not only is that the basis on which we can say that wage slavery is preferable to slavery or that slavery is preferable to cannibalism, but also the basis on which we can say that the society of the freely associated producers is preferable to wage slavery.
Miles thinks it is not possible to judge without "ethnocentrism" (which to my mind is a mistake, because judgements might cut across the ethnos) but if true, would that not mean that we could not say that fascism was worse than democracy, or socialism superior to capitalism.
Marx's morality (which I must concede is open to much debate) was neither wholly relativistic, nor wholly objectivist. He did have a transcendent principle, human development, against which he judged societies, and movements within society. But that principle was itself open-ended, so that in different conditions, what was best for human development was different. As Engels says, pushing the boat out, slavery can be counted an advance on cannibalism.
Furthermore, Marx, I think, did not himself think that he was imposing a standard from without, but rather calling society to account from an immanent standard. That was possible because he thought there was indeed a real development taking place, with an objective manifestation, in the abbreviation of human labour time, which is to say the diminution of the realm of necessity, in favour of the realm of freedom (even where that realm of freedom remained the monopoly of the few, but not beyond the point that it could practicably be generalised).
"It is a well-established fact that the values we hold dear in an industrial society are not shared by people everywhere. For me, the conclusion is inescapable: we cannot assume that the values we hold, no matter how dear they are to us, should be applied as universal standards to rank societies as "better" or "worse". I know this is brazen moral relativism, but consider the alternative: if we arbitrarily say "the values in my society are the universal standards by which all societies should be judged", then we're engaged in equally brazen ethnocentrism."