Side-by-side doc viewing is applicable for desktop-sized (20+") displays, not tablets. Tab displays are already sized-constrained for even single-doc view; SxS-view isn't practical.
Portrait mode for widescreen is likewise constrained. It works for docs with reflowable text, but not for PDFs with mixed graphics/text (ie non-reflowable text) or comics; display width would be lacking for full page views.
I agree that the majority prefer widescreen, mostly because video-watching is by far the prevalent use on mobile devices. I don't agree that widescreen lends itself to productivity in portrait, for the simple fact that Windows hybrids (and Windows 8.1 itself) aren't designed for portrait use. For one, portrait mode means forfeitting use of the keyboard dock for all 2n1s, include Surfaces.
>I have good enough vision I can set the DPI scaling to 100% on Surface 2 / Pro / Pro 2 and still have readable text, which lets me snap windows side-by-side the way I do on my work machine, but apparently that's weird?
Not weird, but an outlier. People normally can't discern text at such dense resolution (assuming default font size), particularly older people. Regardless, for those who prefer it, SxS view work just as well if not better on a squarer display. Split SP3's 3:2 screen in half and you have two 4:3 displays.