I've been reading magazines and books on tablets for a while now, and while I agree that 4:3 portrait is naturally more suited to reading, most magazines are printed at an actual size of A4 paper or larger. So even on a 9.7 inch iPad, the content is being shrunk to "fit to vertical height".

This can make the content harder to read, as you're not viewing it at the actual size it was designed for. The alternative is to read it in landscape mode "Fit to horizontal width", but with visual clarity, the trade-off is having to scroll to read the whole page, which is annoying, and reminiscent of scrolling through a web page.

Yeah, even on the ten inchers there is some shrinking. We would really need a 11-inch 4:3 or something thereabouts.

Although to me, the ideal solution would be auto-cropping (or auto-zooming) the margins. The actual content is often only, say, 9" by 6.5". The tablet bezel serves as a margin for us, we don't need double margins.

The auto-cropping works for most magazines but not all. Businessweek is notorious for using margins to include photo captions, graph legends and related articles. Most mags also use the margins to include page numbers. Source: used to crop margins on magazine PDFs when reading on a 16:10 Android tablet. Not worth the hassle.

3:2 is the aspect ratio for comic books, actually.

I read a lot of digital comics (thank you, Marvel Unlimited), and my Surface Pro 3 is the best device for reading comics I've ever used. The 12" screen is the same exact size and aspect ratio as a physical comic book.