There is no sensible reason in using the various patterns -- such as to "start humbly by saying you might be wrong / you're not sure / ..." -- "to dull the edges of an opinion" when the commenter can simply prefix whatever he is saying with "In my opinion, " and be done with it.
Why call ourselves intelligent beings and rely on such archaic tricks?
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Also, in regards to this pattern being "the root" of a lot of great discourse you have experienced in person (in particular) - what I can say is that I have found such patterns/ tricks to be only necessary when I'm feeling even the slightest malice towards the subject matter or the other person (requiring mellowing down the said feeling); in every other case, a straightforward/ direct stating of the opinion (with little to no adverse feelings involved) has been the most propitious course of action.