While do-able, I think a lot of why people don't is due to complexity, especially at a real-time level. LEDs don't need real time, since people can't observe certain speeds. However, for a stepper-motor, you need to worry about real-time signals, especially when controlling multiple. You really need to be sure that both motors are doing exactly what they are supposed to so your movement is correct and won't damage something. Adding a shift register adds in latency, so the timing becomes more critical.