- It is finished and unchanging.
- Its use of shaders is very minimal, so even lower-end hardware can show the "X-plane 8 model" of lighting.
- X-Plane 8 rendering is completely supported in X-Plane 9. (That is, turn off shaders, and OBJs should look the same in X-Plane 8 and 9.)
- Per-vertex lighting. Lighting is calculated per vertex, and interpolated between vertices.
- Very limited materials. Basically you can use attributes to set emissive lighting (so your day texture stays bright when back-lit, like taxiway signs) and shininess (to induce white specular hilites). The shininess ratio isn't very flexible, but it does match what the built-in ACF shiny property does.
- Very fast vertex output within a batch.
The big weakness of the current situation is that you have to burn a lot of vertices to get close to per-pixel lighting, particularly for very shiny surfaces. I saw at least one plane (I do not recall who authored it) that just had more triangles in the engine nacelles than you could imagine. They look beautiful even in X-Plane 8 - great specular hilites. But that eats into your vertex budget pretty severely - it's not a technique that you could use for every static airplane on a tarmac at LAX.