In modest defense of updating a node:
It makes sense (after a fashion), in a chatty discussion engine (like, say, NNTP, or fark.com) to disallow the editing of postings after they are committed, for the reasons discussed here. But it often occurs, particularly on USENET, that the thread degenerates into a mob attacking some hapless poster who is unable to change what he wrote.
But is it not more to the purpose of the Monastery that we build a collection of the most useful nodes possible? That end would seem to be better served by correcting and improving the original node in response to the discussion. Of course the editing will always reflect the judgment of the editor, with which there will be, naturally, disagreement. Careful composition of each post so that it stands on its own, and can be understood and judged on its own, will alleviate the dependence upon other posts, reduce the appearance of contentiousness, and create a better, more useful node, all at the same time.