> I generally prefer no spatial processing, except in the case of 60s-style hard-panned stereo, which is intended for loudspeakers

Pretty much all audio is mixed for loudspeakers (in a room). It's fine if it is your preference but objectively the vast majority of music, film & etc sounds worse, or at least wrong with headphones. The main problem with these spatialization/binauralization techniques is that they are trying to model something that involves a deeply personal thing, the shape of your ears and head, and the way your brain interprets all that. They usually make these techniques with an average listener "head" in mind but if you are not lucky enough to fall in that average window the effect is likely sound bad or very fake.