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.
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.