Actually while PDF is viewed as the culprit for not being reflowable, the real one is the physical size of the devices we use. For every type of publication there should be a similarly sized device. Page format is important for every work, even on paper...
Since PDFs
are viewable on our readers, shouldn't the true culprit here be our own eyes?
Remember that we're paying for being early adopters and therefore should be patient and flexible for a while. The expandable reader does not exist yet...physically. That little 'size' button takes care of it virtually.
The old motto in my craft applies again. "There is a right tool for every task". In this case every book format
is a tool. Just choose the right one for the right task.