I guess the need for decent high ISO performance depends on what sort of photography you do. When photographing birds I very rarely go below ISO400 but try not to go about 1600 (it's the limit of the camera I use for birding). In a studio situation I stay at ISO100 as light isn't an issue and I want the cleanest images possible as I may need to print big. One time that I can imagine being able to push the ISO really high would be a big plus would be shooting a wedding. In a dim church with no flash allowed, a fast lens and a camera you can trust at 6400 would let you get shots that you would otherwise have missed.
I do wonder where it will end - I rember being thrilled with my 40D as I could use ISO800 without any NR and 1600 with a bit of care, I reckon that the 5D mkII is almost 2 stops better than the 40D. No doubt the 5D mkIIs performance will be eclipsed and in a couple of years we'll be able to use 12800 with great results...
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