@JSWolf - Thanks for the screen size estimate. What I'm trying to do here is get a kind of mental model on what ebook producers, on average, tend to assume about how users will use their ebooks, so your quantification is a helpful data point.
And, FWIW, I also tend to find the <blockquote> to be a convenient shorthand for a specific type of offset text—namely, block quotations (which I can style either at the container-level or at the content-level... or more frequently, both). Whereas <div>s tend not to have any consistent use case; they are just generic containers.