I was reading Ronald Meek's Studies in the Labor Theory of Value the other day. Leaving aside the important criticisms by Martha Campbell, Moise Postone and Geoffrey Pilling, Meek does emphasize that for Marx the theory of value is not primarily a normative defense--as it was for the Ricardian socialists--of workers' struggles over the distribution of the net product. On the basis of the theory of value, Marx attempted to explain the dynamics of capital accumulation.
Or to put it in anthropological terms--and Marx is often best understood in these terms as Paul Mattick Jr argues in Social Knowledge--he was interested as what work counted as productive labor in the historically specific relations of bourgeois society. That what counted as such or what labor produced value created all kinds of logical absurdities and conundrums is not Marx's fault. But then one has to realize the absurdity of our most basic social relations--as Marx always did as Robert Paul Wolff has argued in *Moneybags Must Be So Lucky: On The Literary Structure of Marx's Capital*.
As Geoffrey Kay whom I basically quote here has argued, productive labor as only that labor which produces surplus value is thus not a theoretical definition made for the purpose of studying capitalism, but a practical definition made by capitalism in its modus operandi. Hence valid objections to this practical defintion of productive labor that it is unrealistic (it would be realistic to include all the labor which makes indispensible contributions to surplus value production, eg., domestic work), irrational (labor producing armaments or luxuries can produce surplus value), or inconsistent (a nurse in a private hospital produces surplus value while one in a state hospital does not)--all these valid objections--are properly directed, as G Kay puts it, against capitalism itself. Rakesh Geoffrey Kay, The Economic Theory of the Working Class, St Martin's, 1979 p. 133