Why would Machiavelli, a writer that is now renowned around the world for his realist style, arrange the majority of his works -- including his play -- in the classical rhetorical arrangement of dispositio? 
The culture of Machiavelli's time had a fondness for classical works, many of which praised rhetorical skill. Machiavelli was also a translator of classical texts and fond of rhetoric, especially the works of Cicero. So it could be attributed to the spirit of the times he was in, but it still leaves the question: if he was using rhetoric, who was he trying to convince and what were his aims?
Let's narrow the topic to 'The Prince' . Here you have a book celebrated for 'telling like it is' when it was written with, what appears to be, a two fold aim: (1) Obtaining a political position and; (2) Exhorting the unification of Italy.
I came to this view via two paths:
(1) As a fan of the work itself, and also interpretations of the work, such as Kaplan's chapter on Machiavelli in 'Warrior Politics'. There is something attractive about his works. Machiavelli's realist rhetoric seems 'true' and what could be more truthful than what is real?
(2) Lately I've been reading a lot of rhetoric (for rhetoric class of course), particularly classical Greek and Roman rhetoric such as the Sophists and Cicero.
Combining these two paths and reading other sources have led to a different interpretation of 'The Prince'.
So, what was the context that gave rise to 'The Prince'? Spark notes actually gives a good short overview; but, two points from the spark notes site, that relate to his two fold aim, should be remembered:
(1) "Machiavelli desperately wanted to return to politics. One of his goals in writing The Prince was to win the favor of Lorenzo de’ Medici, then-governor of Florence and the person to whom the book is dedicated; Machiavelli hoped to land an advisory position within the Florentine government."
(2) "The same year that Machiavelli returned to Florence, Italy was invaded by Charles VIII of France—the first of several French invasions that would occur during Machiavelli’s lifetime. These events influenced Machiavelli’s attitudes toward government, forming the backdrop for his later impassioned pleas for Italian unity."
While thousands of words could be written on the rhetorical nature of 'The Prince' I'll narrow the scope again to his letter to Lorenzo de' Medici, which is used as an introduction to the book. I could talk about other rhetorical facets of 'The Prince'. Of the way the first half of the half of the book is pure straight-talking arguments and historical examples, which is the confirmatio. We could also talk about the second half of the book (chapter XV onward) where he shoots down opposing views of Princely qualities and creates his own set of values, which is the refutatio. Or how the last chapter is a classical peroratio (conclusion) where he makes appeals to pathos (emotions) especially the last paragraph with words like love, vengeance, loyalty, tears etc.
But I'll look at the opening letter to Medici.
The primary function of an introduction in classical rhetoric is to build ethos, or credibility with the audience. In this case, Machiavelli is trying to build a connection with Medici.
The very first sentence is an appeal to the cultural memory of a Prince. Namely the experience that Princes have of courtiers offering the 'possessions they value most'. Machiavelli then performs a qualitative stasis shift, a shifting of the 'quality' of the issue, and claims that his gift is different and more valuable: the gift of knowledge won by experience.
Machiavelli also brags about his gift of experience e.g. he has 'very diligently analysed and pondered' and '... over so many years and with so much afflication and peril, have learned and understood'. Bragging about one's personal practical experience (phronesis) is also a form of ethos. It makes Machiavelli sound like an expert on the topic.
Machiavelli then uses another tekne of ethos building: concessio. He concedes that his is work is 'unworthy' yet goes on to say that Medici will like it anyway.
An important tactic related to this valuable gift is eunoia, or having Medici's best interests at heart, after all Machiavelli has not "... embellished or crammed this book with rounded periods or big, impressive words, or with any blandishment or superfluous decoration ..."
The most explicit act of rhetoric in the opening letter is near the end. Machiavelli uses chiasmus (a mirrored statement that swaps the subjects) e.g. "... so, to comprehend fully the nature of people, one must be a prince, and to comprehend fully the nature of the princes one must be an ordinary citizen."
So there you have it. The letter to Medici is an opening salvo to the coming rhetorical bombardment. A piece of rhetoric that was apparently ignored by Medici; nonetheless, it was picked up by others. I shall leave you with two random thoughts on the matter.
Firstly, had Machiavelli's works not been written under the guidance of classical rhetoric would they have been as infamous? This is probably a question for counter-factual historians and might be beyond my own area of interest. I would say the rhetorical style was an important factor in the book gaining ground. There might even be 4GW-slant to his book: a piece of Renaissance agitprop that has had massive influence on politics.
Secondly, realist-style rhetoric isn't all that it seems. It appears truthful because of its heavy use of plain speaking and lack of adornment (he couldn't be hiding anything right? he's just telling it like it is!). Perhaps, but I give you another modern realist-style speaker to think about, who you might not believe at face value. He has been credited by ex-CIA analyst Michael Scheuer as just telling it like it is: he is Osama bin Laden. Here is a speaker who lists a number of 'facts' about bad America is, yet his speeches and statements are classic Islamic rhetoric aka Ilm al-balagha (transliterated as the 'science of eloquence'). Bin Laden's speeches are carefully crafted with his Muslim audiences cultural memory in mind. From the way bin Laden uses cassette tapes in some of his speeches, to his first post 9/11 letter (notice he never mentioned America or 9/11? Instead he praised rioting Pakistani Muslims. After all, bin Laden only has his audience's best interests at heart ...).
But I'll leave a bin Laden rhetorical analysis for another time.
(1) see 'Machiavelli' by Maurizio Viroli, which discusses the rhetorical arrangement of his works. Or this website for a short discussion on the same theme.
(2) I'm using the Penguin Classics version translated by George Bull.
Addendum: Inspired by Purpleslog's post on word frequencies in 'The Prince' here.
Update: As I have been living under a rock lately I just checked my Google Reader and apparently there have been a number of Machiavelli related blog posts in the last week. Mainly due to this WSJ article (via 3quarks daily) and this New Yorker article (via metafilter). Both appear to be well written and interesting. I also found this article on whether Machiavelli's 'The Prince' is a work of satire or not (via a commentator on metafilter).