("reigning in a change")
Many wonder about the practical use of high iso capability, since most are not photographing subjects at dawn, dusk, or at night. It is however typical to be required to compromise between high iso and shutter speed when shooting in low-ambient light conditions that are typical of a stormy afternoon, or on streets heavy with large shadows cast by tall buildings, when using a telephoto lens 200mm and greater... Those are but two areas where the practicality of high iso, is quickly realized.
Using a tripod I would have dropped the iso a stop, and settled for shutter speed a bit slower, but when attempting to get a grab shot, hand held at 300mm, without stabilisation (IS or VR) 3200 iso proved a viable option here... even at 3:00 in the afternoon.
It is real world outdoor shooting especially in the early morning or late afternoon, that makes the 200mm f/2 (canon or nikon) the best outdoor glamour-only lens in my mind; the 300mm f/2.8 a very close 2nd.
I'm really looking forward to the day when 6400iso is good enough to be used commercially in virtually any ambient lighting situation, regardless of the background.
1/400s f/4.0 at 300.0mm iso3200