Hello everyone,
I think the core question in this discussion is not "Can people change other peoples comments so that they contradict the original poster's point of view?" but the question is "Do people actually change other peoples comments so that they misstate the original poster's point of view, and are they successful or are those changes quickly reverted wiki-style?"
The answer to the first question is clearly "yes" which sufficiently explains why many people are afraid of it happening (one side of the posters here). The answer to the second question cannot be given very easily but from my experience the wiki system generally works also in discussions. So can the wiki system be abused? Yes! Is it actually being abused? I don't see it.
The fear of abuse is the prime argument that critics of wikis in general bring forward and the general answer to it is to look at what actually happens and be amazed that it does in fact work by just trusting people and assuming good faith. I feel the same arguments apply to the discussion pages.
So what's my opinion? I am in favor of keeping the discussion pages wiki style, refactorable, editable by all but as Timwi proposed adding a "This comment was last edited on <timestamp> by <username> which will allow other users to easily spot any abuse while keeping all wiki-advantages.
Just my 2c, JP