Where 2.8 comes in handy:
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Photo © Robert Friedlander
I happen to own one of those "2.8 70-200 "fast" lenses" which is the Nikon 70-200 VR
. While I would advise against it you could photograph planes with the lens stepped down to 2.8 to get the higher shutter speeds, but your quality will suffer for it.
I only shoot at 2.8 when conditions prohibit any higher F-stop. For Example when I shot my High School's State final Basketball Game at a local College in their Gym which had poor lighting and I was shooting with out a flash.
You are correct in that you don't need fast $$$ glass to get good pictures, it just makes it easier. Alot of a picture's quality comes from the person behind the camera, not the camera it's self, although these D-SLR help