Its not just about the release of an unfinished novel with lots of rough edges, its our understanding of how the Diskworld evolved in Pterry's mind over the years. TCOM was a simple spoof of swords-and-sorcery epics, but by Going Postal it had become a complex world that fused magic with steampunk. The rough drafts, abandoned dead-ends and private notes that presumably inhabited those disk drives would have provided insights into how this happened.

Update: just read loopbit's response to the same post. Good point.