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.
I rarely do it, "to dull the edges of an opinion". Instead, I do it to provide explicit context (e.g. this my opinion or my observation, similar to the way people will use 'IANAL' in discussion of a legal matter) and to let people know that I genuinely think I'm missing the point. The only way someone might be able to help fill in what I've missed is to explain what I see from my perspective. While most of the time, my perspective turns out to be pretty close to the mark, there have been times when I was really, really off, and someone was able to help provide the context necessary for comprehension.
In this particular case, I actually thought exactly the same thing as the original commentor and am glad to know that I'm not alone in my impression and subsequent confusion that "I think I'm missing the point"!