Very simply, I did the comparison tests because I wanted to see how the images of the two lenses compared. My question, to myself, was "Do I keep the heavier and more expensive 2.8 now that I have purchased the 4.0?"
I was amazed, even in low light, how well the 4.0 compared to the 2.8. However, as was pointed out to me during the course of this discussion, it won't do so well with a moving subject, which I didn't test.
The test help me understand how good the new 4.0/VRIII is. On a day with good light, going to the beach with a dog, for example (see the link I posted) I don't believe there will be any advantage to the heavier 2.8.
The only purpose was to share the comparison and let people draw their own conclusions.