While sub-pixel sampling provides little benefit for diffuse surfaces, sampling the same pixel multiple times does provide better smoothness across the surface

While sub-pixel sampling provides little benefit for diffuse surfaces, sampling the same pixel multiple times does provide better smoothness across the surface of the object. Since it's not much more computationally expensive to do sub-pixel sampling than to repeatedly sample the same point within the pixel you might as well just use sub-pixel sampling. Additionally a pixel has no knowledge of its contents until you cast a ray. And even if the ray did hit a diffuse surface you would have to do more math to determine if the edge of that surface lies within the boundaries of that pixel. Might as well just use sub-pixel sampling.