This question has been asked many times, and when the NEC Panel 5 reviewed the proposals to allow the use as you describe a long time ago, they said "we have got to stop somewhere", they knew there was "no technical reason" though, just that they "did not want to expand on the wiring methods."
Also the "Romex Code Article" says that the EGC is to be used for EGC purposes only.
"Equipment Grounding.
In addition to the insulated conductors, the cable shall be permitted to have an insulated or bare conductor for equipment grounding purposes only."