Light reflections are just one aspect of graphics the Shader Model creates, it is actually used for almost every aspect of producing the graphics (Including geometry). The SM is actually what takes information about object color, lighting, and reflection to calculate the correct color for every pixel on the screen. So as you can imagine, it'd be hard to have effective smoke if your SM was no longer fully supported by your drivers (if it weren't supported at all, you wouldn't see much) as there would be nothing to average the colors to give it that transparent look.
