'Why the systematic denigration of the social value of all "unproductive labor"?'
It should be said that it is not Marx that systematically denigrates the social value of all unproductive labour, but capitalism. In pointing it out that Capitalism fails to put a value on a great deal of work that is socially useful but not profitable, he was condemning, not commending capitalism. This is a case of shooting the messenger.
Incidentally he was always at pains to say that 'to be a productive labourer is a curse' not a compliment under capitalism, since it meant that you were subject to endless attempts to ring more out of your labour. And most of these points, incidentally, are to be found first in the Victorian economist Richard Jones, before Marx. -- James Heartfield The 'Death of the Subject' Explained is available at GBP11.00, plus GBP1.00 p&p from Publications, audacity.org, 8 College Close, Hackney, London, E9 6ER. Make cheques payable to 'Audacity Ltd'