NXT uses multi-threading already in a number of places: loading, shader compilation, particles, and more. (As well as the essential areas like disk IO and audio)
Keep in mind many games have heavy CPU work such as AI and physics that don't apply to RS, so compared to them, we would inherently never use more than a handful of cores.
I'm currently working on multi-threading the rendering side of the engine, as seen in some other modern game engines. It's a pretty big engine overhaul, so likely a good way off, and can't make any promises. That said, early performance results on CPU-bottlenecked machines are looking good!