I just have a couple of comments:
1. Given that this is a sociological journal, I don’t think Heckman should have been surprised that they got a sociologist to discuss his paper. I’m not clear what he means by “world-class credentials.” I think it was generous of Heckman to write the article for Sociological Methodology, and I assume that a primary reason for writing the article was to convey his point of view to the quantitative sociologists. With this in mind, it makes sense that a sociologist be the discussant.
2. The email exchanges are pretty hard to follow. I think this is often the case with email exchanges: everything seems so clear at the time, but later–or to others–it’s difficult to understand what was going on at the time.
For my part, I am glad that Heckman’s article, Sobel’s discussion, and Heckman’s rejoinder finally did get published, since they raise some interesting issues about modeling and causal inference. As I noted earlier, Heckman’s anti-experimentation position is something we rarely see in statistics, so it is good to see it argued (and counter-argued) so forecefully. Michael Sobel is a colleague of mine at Columbia, and I think I did see an earlier draft of his discussion at some point. I would think that it’s good for Heckman, as well as the statistical community, to have these ideas out there, so it all seems to have worked out ok (although with too much trauma to all participants, it appears).