If I may chime in (the guy with the cat picture
), I took that photo at 12800 partly because I was testing it, and also partly because I had 2 or 3 60W tungsten lightbulbs "lighting" (if you can call that) my big living room. Took it with my Nifty 50, 1/125 sec, f/2.2, so there wasn't much light there.
I was happy with the results, especially after I cleaned it up.
Was it necessary to shoot an animate subject at 12800 (she wasn't sitting perfectly still)? Well, I could have shot it at ISO 3200 and about 1/30 and end up with a bunch of motion blur I guess. So all I can say (I may be wrong though) is that I use ISO 12800 when the conditions call for it (for example when I need fast enough shutter speed to achieve a motion blur-less picture). Whether it's a club, or my cat, that's totally irrelevant to me.