But why shoot a Hassy 110mm on a Contax 645 when you can use the legendary Contax 120mm Macro Planar?
There are several reasons as to why a macro lens were not built for portrait lens. I will go into detail if necessary. In addition, here are the other points to consider:
1. The strong background light lead to flares and this is common to the following well known macro lens (Contax 645 Carl Zeiss Planar T* 4.0/120, Hasselblad Makro-Planar T* 4.0/120, Rollei Carl Zeiss Makro-Planar 4.0/120)
2. Digital sensor is quite sensitive to flare prone lens. Keep in mind some of these lens were performing well in film day but on digital it is a different medium.
3. Medium format digital back are best when used in low ISO. Thus, the fast lens are alternative way to keep the balance between high ISO signal/noise ratio and shutter speed especially when doing hand held photography.
4. Finally it has to do with speed, and in medium format F2.0 is significantly faster than F4.0. It is like asking yourself why do people buy F1.0, F1.2, F1.4 lens and not F2.8 lens in the 135 mm format system? One of the essential reason why folks may choose a faster lens over a slower lens is to facilitate the focusing mechanics whether in auto focus or manual focus mode. In low light condition, F2.0 is significantly easier to focus than F4.0. The other significant reason as to why folks choose a faster lens over a slower lens is due to the image characteristic of a fast aperature lens and especially from lens that are capable of wide open sharpness. I believe this is one of the reason why Leica optics are so great and this Carl Zeiss Planar T* 2.0/110 FE is analogous in the medium format system.
-Son