First, zero sharpening in your favorite converter may not have the same effect as zero in my favorite converter. It is more or less a random value. The image you see is computed, and different computations yield different looking images.
Demosaicing aside (think about the B&W Leica), well done sharpening may compensate to some extent for the blurring effect of your vision. There are some publications on that, actually.
Finally, the AA filter softens the image for proper sampling but it starts to blur it well before the Nyquist frequency. Sharpening could restore some of those high frequencies in some selective way. It is based on some a priori assumptions what most images would contain (sharp edges due to physical objects with well defined boundaries), and what most viewers want to see. Think about it as art, not science.
Do the following experiment: take a "perfect" image, and downsize it considerably with some reasonable settings (say, bicubic). The effect of the optics is mostly gone, and since you started with a "perfect" image, it is even sharper now, light? Now, sharpen it a bit. It looks "better". Most people would say that.