heck i dun even recall any situation when i need 6400 least 12800 up. So far just 1600 is already the max i use and it is already pretty useful. Now 6400 being so clean is just godlike for me. Dun even think i need 6400 unless in close to near dark situations and if so i wonder why i am there taking photos anyway.
not really. Actually many indoor lighting require ISO3200 or ISO6400 if you want to shoot at 1/60 (just to make sure there is not motion blur) with f2.8.