I found this gem in the CGtalk forums, couldn't have explained it better.
"Well, when you have multiple lighters working on the same sequence, you ideally don't want them to have to tweak the shaders to get the look that they are after. Not only does this take more time (and sometimes break the pipeline if all the shader parameters aren't promoted up to a settable/animatable level), but it also makes it difficult to keep the look consistent among several artists.
Generally a reference light setup will be made for the show, or for each sequence in a film. The texture artists and look dev artists will make sure the objects look good under those lighting conditions. Then if needed, several variations of the object and/or its shader parameters will be published out for use by the lighting artist. Likewise, a lead lighting artist/TD will generally set up the overall lighting scheme for the set or sequence, and publish that out as well. Then it is up to the individual lighting artists to take the set lighting preset and the surfacing preset, and tweak the lights as needed on a per-shot basis.
Lighters shouldn't be tweaking textures, just like they shouldn't be tweaking geometry. If any texture or shader or geometry problems show up, they should be kicked back as retakes to the appropriate departments. Likewise, lighters shouldn't be tweaking shaders either. Lighters should only be concentrating on the lighting (and often compositing) of the scene.
Now, I'm not saying that in practice many lighters don't wind up tweaking everything under the sun in order to get their shots to work. They do. They just shouldn't have to. It is messy, often hard to reproduce across multiple shots, and generally a waste of their time."