As others have said, it greatly depends upon your subjects.

I have shot quite alot of country sports, often in late afternoon, some even fully at night and they simply don't have the big budget lighting that some city fields have as Norwest mentioned. Basketball played indoors is the same, their lighting leaves alot to be desired. I have shot some of these games for the media and even with the Nikon 400/2.8VR or for indoors 200/2VR wide open at base ISO (200 for Nikon D3/3s/700) I would struggle to get a shutter speed > 1/15, so shooting ISO3200-6400 is very common even with the fastest of glass.
The same applies for many indoor events, weddings etc that take place in dimly lit churches and reception venues.

Wildlife such as birds that need fast shutter speeds to freeze their action are another area as you already noted.

Without these high ISO capable bodies, I simply couldn't do much of what I do and again as Norwest said, having the capability is a game changer for me too. Having the new D800 and all it's monster of pixels requires even more discipline and steady hands when using slower shutter speeds, so being able to crank that ISO a couple of stops is the difference between sharp and a little fuzzy.