Also, this interlude serves to remind us of the complex framing of the narrative and of the fictitiousness of the whole account. Plato frames the story in this way--even making explicit mention of his own absence at Socrates' death--to make it clear that he is not describing the events as they actually took place. Instead, he is using that setting as a powerful backdrop against which he can present some of his own views. In reminding us of this fictitiousness, Plato is also reminding us that we are not reading a story, with the intent to find out "what happened," but rather are reading philosophy, with the intent to think for ourselves and to tease out the truth. In this sense, the interlude also serves as an appeal to closely consider the objections ourselves instead of blindly trusting the opinions of Plato (as conveyed through Socrates).
Socrates gives us four reasons to think that the soul is different from the attunement of an instrument: (1) the soul can exist before the body is made, (2) there are no degrees of soul like there are degrees of attunement, (3) if the attunement argument were correct, it would imply that no souls were better or worse than any other souls, and (4) the soul is master of the body. We should note that Plato clearly has more faith in the Theory of Recollection than in any of his other theories for the immortality of the soul. Simmias' objection is derived from the Argument from Affinity, but this, it seems, cannot stand up to the Theory of Recollection.
A final point of interest is that Socrates' remark, that a frustration with arguments might lead to the position that there is no truth, is a subtle rebuke of the Heraclitean flux doctrine.