It seems to me plain and clear style often serves as an ideal method to give truthful expressions. By contrast, florid and verbose speech/writing often leaves me the impression of dishonesty in expression. In this case, I may take the former style as “good rhetoric” while the latter “bad rhetoric”. But how about an honest person who tries to use florid style to convey truthful meanings? The florid style employed here may not be successful but its original purpose is to convey the truth. If I, as a listener/reader who possess related background knowledge can still understand the noble goal and most of all, the truth behind these florid words, I may still take the speech/writing as a good one. Thus the standard for me to judge rhetoric is not so much of “how it is said” as “what is said”. And I may take words which coincide with my (audience’s) knowledge and appeal to my soul as signs of good rhetoric. In this regard, I find the distinction between rhetoric, knowledge and wisdom in some way blurred, because I tend to take the latter two as a kind of rhetoric too.
Also, in the discussion of orator’s knowledge (259d-261a), Phaedrus mentioned orator need not know “what is really good or noble, but only what will seem so. For that is what persuasion proceeds from, not truth.” But I guess to give most convincing speeches an orator need to know what is true as much as possible. As mentioned above, knowledge and wisdom seem to me a higher form of rhetoric.