I've seen (and made) edits on questions that turned out to be erroneous, either because they accidentally changed the meaning of the question or because they introduced some other error. Sometimes it isn't even an obvious error, and you might want to discuss the change with the person who did it rather than go ahead and risk starting an edit war.
The obvious way to discuss the change is to add comments to the question, but that doesn't really make sense, because the discussion will probably end with that which is being discussed being changed or removed. Maybe the comments will be removed by the people who joined the discussion, but most likely not, and then the question has a bunch of irrelevant comments, making it harder to follow.
Are there other ways to discuss this kind of stuff? I'd recommend being able to add comments amidst the revisions (i.e. they show up among the revisions as separate entries), or to add comments on the revisions themselves.
To show what I would like to avoid, the following question is a good example: