I agree that this would be a helpful addition, especially when looking for particular parts. This way, you could narrow your search. To help limit the size needed on the package, the list could be limited to the brick design, lumping all the different colours together.
A related suggestion could be to publish the parts list on the Lego website. Simple pdf file that would be a copied from the instruction booklet. This could be done if having the list on the package takes away from the space needed for photos of the product and alternate designs.