I had a Voigtlander 25mm f0.95 for my MFT and it was possibly the best lens I've ever owned so I can see the appeal of a specialist lens like that, very wide aperture or macro... but for the slightly more run of the mill stuff if there's a legacy lens available it'll probably be good enough for me.
Of all the legacy lenses I have I think that the only issue is excessive fringing on highlights and edges at the widest apertures with the Zuiko 85mm f2 and FD 85mm f1.8, the Rokkor f2 is much better. Other than that I'm happy with all of the FD, Zuiko and Rokkors I have and even my Rokkor fit Sigma 50mm f2.8 macro is IMVHO astonishingly good.
I can see the appeal of the new MF lenses as the old stuff is getting old and some of the new lenses will no doubt be better built and very lovely but while legacy lenses are relatively readily available I doubt I'll be buying. Even a duff legacy lens can be serviced and if buying for £50 and spending £75 on a good clean up it's still a bargain next to a Loxia
Oh, I forgot... need an adapter too... something between £10 and £100, still a bargain though