OK - let's go back to the circumstances I described above (where I'm shooting at f/2.8, 1/160th and ISO 51,200).
If I switch to an f/4 lens on the same camera, either I will have to lower the shutter speed to 1/80th or raise the ISO to 102,400. If I choose the first, I will get more motion blur in the subject, and if I choose the second I will get reduced dynamic range and increased noise. It doesn't even matter if the lens itself is better; subject movement and digital noise are outside the control of the optics.
This argument is nothing to do with improvements in technology; I'm just presenting the facts of real life as they apply to me today.
If in the future I could change my D3s's for new models which give the same noise and DR performance at one stop higher ISO throughout the range, THEN I would use the f/4 lens like a shot, but right now I am stuck with the weight and bulk of the f/2.8