There are fundamental reasons why lighting at the pixel level is better than at the vertex level.
Imagine having polygons as small as a pixel. The amount of bandwidth required to transfere all those polygons will be enormous.
Imagine trying to create shadow volume silhouettes from models that have polygons the size of a pixel? Very time consuming, and you'll also get lots of cracks and errors.
You'll also get lots of depth errors and fragment flashing just through normal rendering using such small polygons.
There are many other reaons why using very small polys is not the way to go.
A better solution is something like patches etc.. but it's still nicer to light at the fragment level.