A4 & writable would also allow the device to be much more useful for office & academic settings: "fill out this form ..."; "I need to grade these compositions ..."; "I need to read these legal briefs ..."; "let me see that patient's chart ...."
So much professional work is inexorably built around the A4/Letter sized piece of paper. An A4 e-ink device that allowed markup would allow a whole lot more of that to be handled electronically.
In spite of that, however, most books are most definitely not A4, because that's just not a handy size for reading.
There would seem to me to be ample use for both sizes, and probably a few sizes in between.
