I can only speculate as to why the writers of that rule included it, but without a neutral in the switch box, there is no legal or safe method starting a new circuit there. A second switch could be added to operate a new light fixture, for instance, but it needs a neutral.
Too many times a homeowner will use the uninsulated ground wire as a neutral to accomplish what they want, and the result is an ungrounded circuit where there should never be one. Many dimmer switches, which are becoming more popular, require a neutral as well.
So I believe the writers of the electrical code took notice of what was happening out in the field and took steps to eliminate problems down the road, either from electricians too lazy to do the job right or from homeowners that didn't understand just what they were doing. It is true that an extra wire in the switch box can be used as a neutral, but only if the other end can be located and tied to an existing, properly installed neutral. That isn't always an easy task 10 years down the road when no one remembers where all the wires went to! So it is required to be spliced into a true neutral.