personally, i don't see anything but disadvantages for a separate lip UV.  sure you could scale the lip map up for detail, but the idea of having different levels of detail for the lips and the rest of the face seems artistically bizarre.  i literally can't imagine the scenario.

but then, i also hate how the difference in body and face scale means that a really detailed face will show up right next to a completely unacceptable in raw renders neck and shoulder for most portraits. so maybe i'm odd.  on the other hand, i can completely imagine the scenario where that's totally acceptable for 99% of the audience.  i can't imagine the scenario where that's ok on the lips.  i mean, maybe it's just my totally faulty eyes, but i notice as a viewer the weird dissonance of completely detailed lips and kind of fuzzy mole in the previews of SaintFox's awesome textures.  it it were my own work, it would drive me nuts.