Summary: Comments cannot be easily referenced because they are not numbered so if an update must be made, it's confusing and requires one to read through the entire thread. It would be better if an update can be made to the comment that's owned by the person who made it. This would still notify folks and would all be in the same context.
I respect the idea of the integrity of the comment stream but in practice it forces the thread to become longer, harder to compose and harder to read. In this specific case I wanted to add a more up to date version of an attachment that I had associated with an earlier comment but could not do so. Instead I have to create a brand new comment and refer back saying this is an update to the earlier file I added to comment ("so & so"). This is made harder by the fact that the comments are not numbered, so there is no easy way to refer to them. They have hyperlink references but these take you to a new page so are not that useful for in page references.
It's currently possible to edit previous comments that you have created. You can hover on the text of a previous comment made to make any relevant revisions.