I have a little example of the difference shading can make even when the shape has a model.

The brighter green coach on the left is the default DT issue; the bodyside is all one colour, and you can see it has a little curvature. The one on the right - beside being a bit darker - has had a little graduated shading applied to the lower curve of the body. Not very much, you can hardly see it if you look at the actual texture, but the difference it makes is enormous.