Ken,
If you use a ground wire for neutral current and a ground fault
happens, It may not be big enough to carry both ground and neutral
currents. Also, it is more likely for such a combined wire
tooverheat/corrode/oxidise and end up with a high resistance connection
somewhere allong its length. Then it wouldn't work right and someone
would be dead! (Maybe You!!) The code is there as a safety code to
protect people and property, not just to keep inspectors employed. It
may save your life someday, in fact maybe it already did, you just
don't know it. It also keeps the lawyers from having too much work.
:-)

Stretch