Sports and events. Even professional arenas and venues have poor lighting for the situation. In a well lit arena, I was still lucky to get ISO 3200 (at 1/1,000 2.8)
But smaller venues, high school sports, and dimly lit churches? 6,400 or 12,800 was normal. Sometimes even with a prime. And you had to nail that exposure or the image would look like pointillism.
Nowadays? Between DXO (amazing), Topaz (great for jpegs), or Adobe Denoise (built into ACR, and solid), we have it easy.
I've gone and looked at some of my old RAW files from a Canon 1D (2001 camera, 4mp) and a 1D II (2004 camera, 8mp) and its night and day how well they process and blow up now. What's funny is that I thought the same thing 15-20 years ago and it was true because we all knew how "good" a file could be processed, so we accepted the levels of grain.
I still shoot with a pair of D3s (2009, 12mp) and it works great. Running files through DXO cleans them up, but my clients didn't complain when I didn't use it, so it's really more for me to look at and go ooh and aah at the technology.
I will shoot at whatever ISO gets me the image that I need to get. Would I love to shoot everything at ISO 100 and freeze motion and get the lighting and colors I want? sure. But that would be too easy.
As someone who has shot for a D1 program, all this tracks, but:
that one really hit hard. Happens to me all the time.
Great read