I wonder if Plato believes that Socrates's speech A (preferring non-lovers) is technically equal to Socrates's speech B (preferring lovers). That is, I wonder if Truth is not only a moral but also a technical necessity. John C. Adams argues that Socrates's speech A is "technically correct, but morally depraved as far is it celebrates the virtue of being an evil lover in a more rhetorically effective manner than Lysias' speech" (8). And while I find this language useful, I wonder whether the issue is not morality--that Plato is not simply claiming that Socrates would be impious to deliver the speech preferring non-lovers to lovers, but additionally that he would be a poor rhetor.
Truth is not a side issue for Plato, as far as I can tell. Socrates in fact asks Phaedrus if a rhetor needs to "know the truth about the matters of which he is to speak" (156). Phaedrus has heard otherwise, but Socrates seems to make the point that rhetoric needs Truth even to be technically good. I cannot truly say that an argument is good if it comes to a conclusion I know to be false. It might be interesting or pretty, but to be rhetorically good it must also be True.
Adams, John C. "The Rhetorical Significance of the Conversion of the Lover's Soul in Plato's Phaedrus." Rhetoric Society Quarterly 26.3 (1996): 7-16.
Plato. Phaedrus. Trans. H. N. Fowler. The Rhetorical Tradition: Readings from Classical Ties to the Present. Ed. Patricia Bizzell and Bruce Herzberg. Boston: Bedford/St. Martins, 2001. 138-168.