no, because most people don't use the separate lip material.  they generally have the same exact material for the face as for the lips.  you can have a different mat zone for each poly, and if they all take the same UV map, and you give them the same material, it's just a pain to manage.

the most i've seen it do is have a different specular and bump, due to lazy specular and bump map creation (take the diffuse map, greyscale it, take the greyscale, invert it to account for eyebrows, leave it right wise for lips).  but that's taking some chances in any case.