I feel that the aperture to use is personal preference..
Some might only want the eyes to be sharp, some might want the whole face to be sharp. Some pple feel the bokeh is very important, some pple feel it's more important to have the details and the hair style all in sharp focus.
Artistically, maybe a shallower DOF creates a "nicer" picture.. but not all photographers are "artist". To some pple, a picture of a person where half the face is cropped away may look very nice and artistic, but to another, not showing the whole face is simply bad portrait photography.. this is similar to whether getting the 2 eyes in focus or the whole face (including the ears) in focus.. it's all a matter of personal preferences and style.
Having a F1.2 lens gives added flexibility of thin DOF, but if the photographer prefers a sharper picture in some situations, the max aperture may not be used. How sharp is "enough'" is subjective. To some pple, F1.2 sharpness is enough, to another, if they can have it sharper at F2 or F2.8, why settle for less? But they still get the expensive lens if they can afford it for extra flexibility.
Just my 2cts..