Most material properties are nearly constant as the view direction or lighting changes. However, the metal shading presented by Gooch et al. is the replication of the anisotropic reflection  due to the surface of an object and the reflection of the environment. When a real metal part is rotated in one's hand, the banding does not stick to the object, but remains constant since the environment is not changing. However, in an interactive environment it may be too jarring to have the metal shading change abruptly. Using a metal texture would be more appropriate and a metal texture in an interactive environment would still properly convey the material property.