Look here, and ant the two photos of the lady in front of the blue background (search for "sagittal") http://www.imx.nl/photosite/zeiss/test85/t004.html
Those two pictures look separated by 3 stops, and I don't think that such an experienced reviewer as Erwin Puts would have compared them if they were 3 stops different.
Also, I never wondered about portrait lenses, I wondered about extreme wides - eg a 28 mm gaussian and a 28mm retrofocus used with an MF digital back. In digital, as we all know, lane of focus effects are considerably magnified with respect to film !
My original post specifically mentioned portrait lenses, and the much discussed need for large apertures for the sake of extremes of background blur in portraiture. So I will stay with that topics, as in your Sonnar 85/Summicron 90 "woman in blue" examples.
In those photos, the most likely explanation is the combination of substantially greater aberration blurring in the Sonnar (as Ray has suggested?) and the different perspective: the Sonnar is clearly closer to the subject, as the woman's image is larger despite the slightly shorter focal length. And the Sonnar image seems less sharp overall, on the woman and dress as well as the background, which is more evidence for its having significantly greater aberrations (or incorrect focus). Reducing focus distance will increase background OOF effects.
There is no doubt that under-corrected aberrations can soften backgrounds (and even main subjects) such as in "soft-focus" lens designs, in which under-correction of some aberrations is a deliberate design feature. Such design choices are irrelevant to my initial curiosity of comparing MF to 35mm format options, since soft-focus lenses exist for 35mm format too. Positioning of the aperture diaphragm can also affect the appearance of background blur, making the large "disks of confusion" harder or softer at the edges: I believe that macro and portrait lens designs often take opposite approaches on diaphragm placement, favoring maximum sharpness versus "nice bokeh" (softer backgrounds) respectively.
But in the "woman in blue" examples, the Sonnar 85 disks are clearly far larger, not just softer.
If such design differences could adequately soften distracting backgrounds and reduce DOF, why would photographers bother with f/1.2 or f/1.4 lenses for that purpose (as opposed to the main large aperture purpose of higher speed); they could just use soft focus lenses, some of which allow turning the "softness" on or off, or at least increasing and decreasing it. Probably there is more to the desire for "background softening" than that.