Carl Trueman is back at it again with a thoughtful post listing three reasons why 19th century southern theologians are not worth one's time (along with a reference to my long-lost brother, Rob Lowe). While admitting that perhaps they aren't the most useful when it comes to systematic theology (a debatable point--Dabney, for example, was the theologian who best convinced me about the biblical and theological rationale for infant baptism), I would submit that it is exactly on the issue of how theology operates within cultural systems that Dabney and Thornwell prove most useful. That is to say, Dabney and Thornwell are important for historical reasons (and contemporary lessons), even when they may fail our tests for theological purposes.
I think this was one of the points that I tried to weave throughout my biography on Dabney. It was why I used this Dabney quote as the epigraph for the entire book: "We shall be wise, therefore, if we harken to the striking instruction of these instances, and make it our method to submit with modesty to the sober teachings of the past in all our legislation for the future." All too often, we have a hard time thinking self-critically about how our theological claims serve to legitimate (illegitimately!) various familial, economic, political, social, moral choices we make. By looking critically at someone like Dabney or Thornwell who blew it so royally on race and slavery, we have a better opportunity for noticing our own blind spots in our cultural systems.
I've made this point before from the perspective of cultural history (here ; here, here, here, and here; as well as here and here)--but it bears repeating: we can only gain perpsective on our present cultural systems through a critical appraisal of the way others have lived in cultural systems in the past. But even more, it is only as we view this past critically and sympathetically do we really understand that all human beings are deeply and profoundly sinful and flawed, save one: Jesus is the only hero of the historical story.