Thread mode has virtues as well as vices. (see also ThreadModeConsideredHarmful)
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Signing statements allows people to take responsibility for their beliefs and statements. Somebody who signs a posting is putting reputation at stake. By contrast, anonymous posting is a favored tool of the flamer, not only of the WikiMaster.
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Signing statements helps to create a context. When you know that the same person said, on different pages, "Software is evil" and "All technology is dehumanizing", you have a better understanding of the motives behind the first statement.
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Signing statements can lend them authority. There are a couple of Wikists whom I have come to consider wise. When I see an anonymous "Software is evil", it has no impact. If I saw "SoftwareIsEvil" signed AlistairCockburn (substitute your favorite Wikist), I'd have to stop and think hard about my prejudices.
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Signing statements preserves the individual character of the statement. "I dislike working in troff" signed BetsyHanesPerry is clearly an individual foible. An anonymous "troff is a nuisance" is a general statement, and doesn't invite dialogue, because there's no personality on the other side of the conversation.
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Signatures preserve DramaticIdentity, which makes ideas that run through more than a single page comprehensible.
In my opinion thread mode is good for brainstorming ideas between people. When the brainstorming slows after a while, then is perhaps a good time to clean it up. I was taught that when collecting ideas, don't worry about organization at that time. Organization comes after the collection phase is mostly complete and sifting and grouping starts. Don't sort your fish while you are still fishing.