David Hull said:
With regard to the 400k ISO, IMO the real question should be "where does the ability of the AF to work crap out?" Super high ISO isn't of much use if you can't focus clearly.
I've seen this comment by others also and one answer could be, and this goes for me, it's not about the pitch black use of it, it's about shutter speeds. I often use ISO 5000 outdoor in not too low light to get my 1/1000s-1/4000s shutters. When shooting sports indoors and you want some dof for example, it makes sense to have a very high iso. Have I ever needed above 51k on my 1dx, no, but I like to shoot with the widest of apertures.