Rhetorical Diagram

Here's Longaker and Walker's diagram of rhetoric:

We have rhetors creating texts, images of themselves, and intended audiences for those texts within particular occasions and given particular presuppositions. We also have readers interpreting/recreating those same texts/intended audiences/implied rhetors.

The only objection I have to their illustration is that Occassion and Presuppositions are here tied to the text, as if the text itself inhabited a specific occasion and was tied to a given set of presuppositions. I would suggest two things: first, that it is the actual rhetor and actual reader, rather than the text, who exist in particular occasions and are tied to particular presuppositions, and second, that because some forms of text can exist over long periods of time and can travel from place to place, the occasion and presuppositions of actual rhetor and actual reader may be quite different. The Actual reader's occasion and presuppositions will always be temporally later, and will sometimes (not always) be inherited and altered versions of the actual rhetor's occasion and presuppositions (as in the case of an author and reader who exist in the "same" culture 100 years apart).

So I'd prefer to draw the diagram like this:

I guess my first question is: How useful is it to have a model like this? Is it misleading? It seems to give the impression that rhetoric is a fully known and map-able object or system. Rhetoric itself is in flux, right? It changes over time. So what purpose do such diagrams serve? Can we use them for something other than feeling like we've got it all figured out?

Second question: In chapter 3, Longaker and Walker say this: "Though kairos is arguably most important, the better part of classical and contemporary rhetorical theory focuses on content and form, not timing." Why is that? If kairos is the more important feature, why isn't it given its due? My sense is that examining social situations forces one to move away from the rhetorical formalism expressed in diagrams like those above and toward something more like ethnography. Situations are always particular and don't lend themselves well to strict formalist frames. Are there examples of a rhetorical method that is fully (or at least more heavily) focused on kairos rather than content and form?

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