Really fast lenses are a manufacturing/lens designer vanity. I do wonder that beyond F1.4 what real world advantage is afforded? Given that both size, weight and filter size/no filter come along with the sub F1.2 lenses they are of little interest to 99.9% of photographers. I could look at my 70,000 plus image collections and probably find only a handful of times I have sought to use below F2.8 due to DOF (depth of field) limitations.
I think Sigma with their ART series of lenses have taken a clever path between optical quality/sensible but fast lens speed and size/weight and justifiably earned a large share of the market. Time for Sigma to bring out a 300mm F4 with builtin teleconverter
It entirely depends on what and how you shoot. Taking weddings as an example – if you light your weddings then fast lenses aren't as important, but if you're delivering images to clients in dimly lit venues where lighting isn't possible then an f/1.2 aperture certainly makes all the difference!
On the subject of depth of field limitations, it's worth bearing in mind that faster apertures on crop sensor cameras have the best of both worlds in this respect. On Micro Four Thirds, for example, the same 2x crop factor that doubles effective focal length also doubles the effective depth of field – this means that an f/1.2 lens on an Olympus camera still gathers light like a f/1.2 lens, but has DOF equivalent to an f/2.4 lens. When shooting weddings or events, this gives you the low light performance with a much more practical depth of field.
Agreed that some of these ultra-fast lenses are very much showcase optics more than anything – look no further than the Noct f/0.95 for that!