[JJ_SO]

Probably partly due to the very narrow E-mount....
https://www.lenstip.com/566.8-Lens_revie...tting.html

So, it does not reach anywhere near f1.4 light levels even, so no use getting it to be able to shoot in lower light. And not very nice bokeh (a "pulled apart" quality to it, probably partly due to the huge vignetting), so... It kinda misses both of the f1.2 points?

That is a misconception from you. f/1.2 or f/1.4 always shows vignetting, no matter how big the mount-Ø is. And most of it does also come from the microlens geometry of the sensor which can't counteract against normal optical rules, bit on the contrary is amplifying them. 

And as there is no other lens to shoot at low light, there's not much of a choice - no one will put the main subject into the corners and no one will take reproductions of paintings or whatever with such a lens(brick). It is meant for desperate need of the last bit of photons coming in, and it is also meant to play with superthin DoF - and knowing Sigma's approach, it is also meant to show off what they can do - but it can't be a match for Canon's 85/1.2 approach(es) on their R mount. And what Nikon can do in terms of big mount (mouth as well...where's the f/0.95?) still has to be seen

There are other lenses with "flaws" which are producing unique pictures. I don't see the Sigma 35/1.2 as a close to perfect lens, but as one with a certain character - you like it or not.
I don't see much of a benefit from f/1.2 to f/1.4, here we can agree, but there were some shots on DPReview's samples I could not do with any of my f/1.4 lenses.