if you define rhetoric as persuasion, false, showy, artificial... "Rhetoric, that powerful instrument of error and deceit" John Locke, Essay Concerning Human Understanding.
I'm just starting to read around rhetoric (and its application to design research) - seems to be (appropriately) a huge range of arguments around the value of rhetoric as a field of study at all, let alone for design. The value of rhetoric for design is that studies persuasion - how do you communicate a message in the most convincing manner. This the fundamental purpose of visual communications.
The central concern of rhetoric has always been method and manner: how to discover the most effective way to express a thought in a given situation, and then how to alter its expression to suit different situations. http://lsb.scu.edu/%7Eemcquarrie/rhetjcr.htm