The benefits of instancing are felt more keenly in ray tracing because of the need to keep geometry in memory. If you're just dealing with direct lighting it's

The benefits of instancing are felt more keenly in ray tracing because of the need to keep geometry in memory. If you're just dealing with direct lighting it's very easy to load and discard geometry as required as you rasterize the frame. One you start casting secondary rays you end up having to keep more of the scene in memory in order to shade a particular surface, since the lighting contribution from other geo can't be localised.