Exactly: expansion of an existing argument is comment-worthy, but significantly rarer.

I'm missing a coherent discussion, instead I'm often stuck in a game of block & attack. One young hacker wants to show the other young hacker that he is superior. Personally, I find this is just noise... I seek comprehension.

Coherent discussion IS a sequence of block and attack--thesis and antithesis--ideally (although rarely in practice) ending in some kind of resolution (synthesis). Comprehension without disagreement does not necessitate discussion or further remark; the best sign that you've written something of value that others have comprehended is when a comment is voted up without replies (the upvotes indicate value, and the lack of reply indicates agreement). Any other system would result in the overwhelming additional noise of a cacophony of "I agree"s with possible small variations.

In practice there aren't any totally novel ideas. So, if you gotta start every discussion from scratch with block & attack it gets tedious. How would a jazz musician be able to jam with new guys if they don't agree on something? I guess if hackers were Jazz musicians, the audience would hear short staccato outbursts mixed with gaps of silence.

And then there is the difference between an elegant attack, which you can learn from, or the usual trying to misinterpret a statement in their favor, which is called framing.

Continuing your musical analogy, a "discussion" without disagreement would be like a musical piece in which every instrument played precisely the same notes and every singer sang precisely the same words at the same time. In such a circumstance, the presence of additional people is largely irrelevant.

A sequence of arguments going back and forth between two people, on the other hand, are essentially the back and forth of a duet playing separate themes (that, ideally, mix at the end). While certain sorts of arguments are non- or counter-productive (analogous to internally discordant themes), the possibility of such things occurring does not imply that this mode is not superior to single-view discourse.

The fact that discussions center on points of disagreement is not an accident, and does not imply that there is not agreement regarding most things. Disagreements are simply the portion of a topic around which someone believes that there is more information that an outside observer should consider when formulating their beliefs, whereas when there is agreement no further information is necessary, so there is no value in transmitting more signal.