The math of DoF:
Say if we prefer 4:3 format, then we get 32x24mm from 135 and 54x41 from 645, which is 40mm diagonal vs 67mm, factor 1.7. To match the look of an 80/2.8 medium format lens would require a 80/1.7 = 47mm and 2.8/1.7 = f/1.6, closest typical real lens would be a 50/1.4.
The 135 format has lenses with very short depth of field, so I don't think anyone uses MFD to get shorter, as you probably can get even shorter with 135. To match Canon's 85mm f/1.2 you'd need 150mm f/2 and few (any?) medium format systems have that.
Ultra-short DoF is rather gimmicky though and doesn't make good portraits, so it's not a loss. I've heard that many prefer the bokeh of MF lenses, and that there are some finer smoother quality in the transition from in focus to out of focus. I don't think that has to do with the format, but possibly due to that you have simpler lens designs. The high res ultra-wide aperture 135 lenses are certainly not simple designs, and maybe some aspects of the bokeh suffers from that.
A big drawback with most MFD lenses as I see it is that they use apertures that are not round, leading to ugly bokeh highlights. It's a problem on my tech camera too, due to the Copal shutter, fortunately I don't use bokeh often in my shooting style but if I would I would not be happy about that. All high end 135 lenses have round apertures.
With the newest high end 135 lenses designed for high res sensors like Otus and Canon's new 35mm/1.4 I've noted a real improvement in bokeh, bokeh fringing is almost completely gone. While 135 format is still overall lagging behind in resolving power due to the lenses, these new lenses is changing the landscape.
I think that the choice of MFD of the SLR type will be less about resolution or depth of field in the future, but more about if you like the camera ergonomics, and the raw converter workflow and the rendering (color etc) you get in that workflow. It remains to be seen how well the current pricing model will work for the new landscape. I think we will see the system come closer also in price, the high res 135 is already notably more expensive than earlier (due to increased lens complexity), and we have seen lower MFD prices here and there, like the new Leica S.