I think that we are ignoring the role of artwork as a type of social commentary. Although it isn't very useful out of context(except for a historical perspective), it does have life in its own time. Take for instance the anti-natzi artwork in the netherlands, or the pro-communist artwork of the soviet union (often with hidden anti-communist sentiments).
All though I haven't seen any of Witkin's work, I am guessing that he is challenging the unhealthy and somewhat bizzare judeo-christian views on death that is prevelant in america.
If everyone was only interested in making 'Great Art' and 'Great Music', the world would be a pretty boring and stiff place. Sometimes it's more fun to play with our inadaquecies than with the idea that we are made in God's image.