First of all, each page of a book must have an identifier, whether it's printed or not, including the internal frontispiece.
What this identifier should be is really unimportant, so long as it helps into navigating the book. In most books, there are different styles for these identifiers: Roman numerals for the front matter and standard numerals for the rest of the book.
The reason is inertia. This method for numbering pages is as old as typography and was born to solve a very practical problem.
After the approval of a manuscript for publication, the publisher passed it to a copy editor, who fixed errors and style, then handed it over to a typographer. The manuscript was annotated in such a way that the typographer could respect the publisher's style.
Next, front matter would be prepared. The author would write an introduction, for instance, or a preface would be requested to someone else. In subsequent editions of the book, more introductions or prefaces might be added.
What's the problem, then? That it is unknown how many pages the front matter would consist of, until it's in final form. It is essentially impossible to know this number in advance. Numbering pages in the main matter after having typeset the front matter would be impossible as well: cross-references would become wrong, for instance. In really old times, the preparation of the main matter required the longest time and folios were normally already printed when the front matter arrived.
Using a separate numbering style solved the problem. Well, not completely, because the front matter could not exceed 16 pages (how many were printed on a single sheet of paper for later cutting). This is the reason why sometimes you find blank initial pages.
The problem doesn't present with the “back” matter. And what numbering style would be used? I've unfortunately seen institutions that require the back matter to take on Roman numerals from where they ended in the front matter! Does this help in navigating the book? Not at all! Where on earth will page xxxiv be? At the beginning or at the end?
Uppercase Roman numerals? Why? There is no reason whatsoever.
Of course, with modern computer typesetting it's really easy to have continuous numbering from page 1 to the end, without distinguishing between front matter and the rest.
How do you do it in the
book class? Simple: modify the declarations to be
and you're done.
My friend Ivan Valbusa's class
suftesi does this and doesn't use Roman page numbers anywhere (we discussed about the problem). In my opinion, every class for book publications should at least provide an option for this style. Traditions are not to be blindly obeyed.