Acknowledging that slaves had real economic value is in no way a declaration of support for slavery. The fact that you can make such a jump in logic is profoundly disturbing to me. Those sorts of conclusions take away from honest discussion.
(So an anarchonistic example is that a car you own will remain in better shape than one you rent. That's actually not such a terrible argument, though of course we'd reject it nowadays.)
That said, I don't personally know either way, and Wikipedia does mention that _Uncle Tom's Cabin_ is popularly seen as propaganda. (I personally haven't read the book, but it wouldn't surprise me if it were 'propaganda' in the sense that it was biased and promoted a political point of view. Apparently, before the Nazis, propaganda didn't have such bad connotations.) http://en.wikipedia.org/wiki/Uncle_Tom%27s_Cabin#Literary_si...