What Rob said.
If all the sides shared vertices, the lighting gets smoothed across the edges, instead of giving a sharp, flat-shaded appearance. You can see this by doing UpdateNormals on a cube to view the awfulness.
If all the sides shared vertices, the lighting gets smoothed across the edges, instead of giving a sharp, flat-shaded appearance. You can see this by doing UpdateNormals on a cube to view the awfulness.