I think another reason why "southern" Presbyterianism is worth time and study is because it focuses attention on regional difference and the power of nationalism, and so complicates our understanding of change over time. To focus on the American South and to note the differences in religious development in that particular region provides a wonderful laboratory for understanding these issues.
The American South's continued regional persistance has been a major theme of US historiography. As James Cobb's recent Away Down South demonstrates, the American South has typically defined itself against "the North" (or against "damn Yankees"), while northerners have viewed themselves as "Americans" and southerners as recalcitrant against assimilating into the triumph American (read "northern") story.
This process of identification was especially pointed in the two great attempts to "reconstruct" the South--the period from 1865-77 and from 1945-65. Ironically, these attempts to force southerners to a more just society ultimately reinforced the abiding alienation that southerners felt toward "Americans" and perpetuated issues related to continued regional identity.
The one sociologist who has spent the most time explaining the South's regional persistance is John Shelton Reed. He has argued that there are aspects of southern regionalism that have rightly died--especially, issues related to social justice for southern African Americans. But there are continued aspects of regional particularly that mark the American South as a different place--foodways, sports and hobbies, ideals of honor and masculinity, music.
And religion. That is where the entire conversation comes back around to southern Presbyterianism. This is the case because Presbyterianism does develop somewhat different in the American South than it does in other parts of the country--for one thing, it is smaller; it is restricted to the 11 original states of the Confederacy; it wrestles with issues related to one-party politics and Jim Crow; (ironically) it demonstrates great denominational loyalty than northern Presbyterianism; and it experiences greater interaction with religious "others" (esp. Pentecostalism and Disciples of Christ). I'm sure there are other things that will occur to me on this point; and I may come back to revise this list.
Viewing Presbyterianism within a self-contained region should assist in understanding the relationship among religion and other factors within a cultural system. In the end, it should assist believers in thinking through religious truth claims with greater self-criticsm and humility for the good of the Kingdom and the church.