Thanks for the reply, Nasim! Generally, I agree with you about the performance of newer lenses. But in my experience the difference between the 28mm 2.8 ai-s and the 28mm 1.8G is not as clear cut as one might think.
I have owned both the 28mm 2.8 ai-s and the 28mm 1.8G (sample size of one each, though!). Focused at longer distances, the 1.8G was certainly a superior performer, especially in edge-to-edge sharpness. But focused at close-to-medium range, the ai-s held its' own and actually seemed a bit better in the area between the center and the edges.
So I do agree that as an all-purpose lens the 1.8G is a better bet, especially since the price gap between the two has narrowed significantly. But if you don't use the 28mm focal length much at long range to infinity, the old ai-s does seem to slightly outperform it.
BTW, a friend of mine at a camera store told me a couple days ago why some of the old Ais lenses (especially those considered "classics") have become pricy. It's videographers with mount adapters who are creating the demand for good manual focus lenses with aperture rings in the wide to short telephoto range - they just "de-click" the aperture ring.