I have worked on several hundred of these types of contract processes in the last few years, and you are absolutely correct: sequential is where it's at for these situations. I have, however, encountered a few situations where time was an issue so I had two or three different versions out to different parties at the same time, and then merged the proposed changes where possible and sent out alternate versions where the changes differed. That process was... not fun, and could definitely use a more coherent workflow than manual merges or Word's built in merge features.

But the workflow only "looks" sequential, isn't it? In many stories told here a user may have inadvertently revert clauses to old versions and this can be missed. This happens because it is also an asynchronous work.