While batching (and reordering) can have a benefit overall for rendering, it can have a significant (depending on how thoroughly you do it) overhead, and Disney

While batching (and reordering) can have a benefit overall for rendering, it can have a significant (depending on how thoroughly you do it) overhead, and Disney are primarily doing it because they use Ptex to do texturing, instead of the more conventional and more widely used UDIM texture atlasing method for assigning textures to meshes. Ptex really chokes with non-coherant texture accesses (partly due to the anisotropic filtering method it uses) in terms of thread scalability, so Disney have gone to great lengths to get around this issue. Doing such accurate texture filtering (the same goes for standard UV EWA filtering) is technically better, but is expensive, especially in a path-tracing context, where the whole point is to amortise the cost of shading over all the rays by making each intersection / shading calculation as cheap as possible. From what I hear there are big downsides to this current implementation: the streaming of batches is pretty much off-line, so time till "first pixel" is significant, and thus you don't get interactive rendering functionality.