I may invite flames with this remark, but I think Nikon feels f1.2 to be more prestige than anything because the difference between 1.4 and 1.2 is a mere 1/3rd stop. Nikon, being the practical lens manufacturer has chosen to not waste time on too many of these prestige lenses and focus more on "real" lenses that photographers can afford to buy and use on a daily basis.
:sweat:
For some, the bigger the better.. Nikon is targetting the pros who needs to get the picture, not the collectors who just wants to show off their equipment and their associated prices. How many times do people shoot at f/1.2? I have several f/1.4 lenses but I hardly shoot at f/1.4 unless I need the shallow DoF.
Practicality wise, the DoF at f/1.4 is too shallow for many purposes. Focusing becomes a hit and miss thing and the tendency to miss is higher than getting a hit. In studio applications, the flash are usually too bright for f/1.4. So? Either stop down the lens or use ND filters. Daytime shots, even at ISO100, shutter speed is like 1/1000 or higher.
The only reason fast lenses are good is lowlight applications. Then the spherical aberration kicks in at that kind of aperture which renders the image not as sharp. f/1.2 and f/1.4 may only be a third of a stop but in long exposure, a stop can be the difference between a 15 minute exposure and a 30 minute exposure. But that's for very specialised applications. More often than not, half a stop is almost negligible for many.
For general night photography, DoF becomes an issue again. Usually, I shoot at f/2.0 and opening up to f/1.4 only when there is no other choice or I want a shallow DoF effect. The DoF at f/1.4 is already very shallow, at f/1.2 it's razor thin. I try to keep my shutter speed around 1/30 and 1/15 using ISO to compensate (I love digital, don't you?.. ;p)
I like Nikon's philosophy. When you want to build something, build it good. No point having a lens, say, with an aperture of f/2.8 when it's useless. That's why Nikon rather build kit lenses with maximum aperture of f/3.5 and having decent quality at f/3.5 and not only after stopping down a stop or 2 like many of the competition.
Yes, it may be expensive to build f/2.8 zooms to be perfect but for people who really need it, they will buy it.