PDFs and comics are not a small use case at all - the push to larger screens is mostly driven by people who want to read scientific papers, business documents, etc which come in pdf form, or manga and other graphical works (the drive for color ereaders seems to come almost entirely from this segment). The smaller "ebook only" readers are much cheaper and marketed less aggressively.
Even optimized e-ink devices are not particularly suitable for those kinds of materials and most of the e-ink reader manufacturers have not tried to address that market. It is generally better served by LCD tablets.
