The application of explicit UV coordinates is a standard practice in 3D and a must for complex models in VET.

Imagine that you want to create a spotted sphere with a face on the front of it. You will need explicit UV coordinates for the spotted section of the sphere and explicit UV coordinates for the face section of the sphere (see image at http://solutions.viewpoint.com/documents.html#3). All three models in the illustration (sphere 1, 2 and 3) have the same two texture maps applied to them (texture 1 and 2).

Sphere 1 has no UV coordinates and only diffuse colors are displayed.

Sphere 2 has one set of UV coordinates applied to the entire sphere so both textures get wrapped to the entire sphere displaying only a portion of texture 2.

Sphere 3 has UV coordinates for the spotted sphere and a separate set of UV coordinates for the area of the sphere that the face is mapped to.