I hit this case while looking at a bug in X-Plane 930; for some reason the normal matrix produces junk results when used from a vertex (but not fragment) shader. Totally strange. The nice thing about using the model-view matrix is that it gives me a work-around that is theoretically a faster path. (The alternative of using the normal matrix in the fragment shader means slower per-pixel operations.)
(See FAQ 5.27 for a traditional explanation of why you'd use the inverse-transpose matrix on normals.)
We actually don't have scaling at all on our model view matrix for another reason: fast sphere culling. Basically to cull a sphere you need to:
- Transform it from model-view to eye space.
- Find the distance from the sphere to the six faces of the viewing volume.
So in X-Plane, we don't ever scale the model view matrix. We don't offer it as a content option for authors (not that it would be useful anyway) and we do our zooming by manipulating FOV.
The side-effect of this is that we could use the transpose instead of inverse if we need to undo camera rotations, and we don't need to ue the inverse-transpose to transform normals.