It is difficult (if not practically impossible) to determine if two users have participated in the same discussion. We can easily determine if two users have talked on the same talk page, but determining if they are in the same discussion is extremely difficult (especially if they did not leave an accurate signature). It is also difficult to determine if one user modified another users comment.
StructuredDiscussions provides a way to store discussions in a structured manner so they can be queried easily. We should use this on wikis that do not use Structured Discussions as a storage/retrieval mechanism. The UI (and workflow) would not change for users, we would only parse the revision and store it within Structured Discussions. This would allow us to quickly determine if a user has participated in the same discussion, or if they modified another users' comments (etc.)