Its just a matter of safety. The neutral conductor carries the normal "return" current, or "unbalanced" current, depending on the context. The grounding conductor is only there to conduct current during an accidental "hot-to-ground" contact. That is, if a wire gets pinched or damaged or comes loose and touches the metal enclosure, and that metal enclosure is bonded with a grounding conductor, the resulting ground fault current will be assuredly conducted back to the neutral at the service, and complete the fault current loop necessary to trip breakers or blow fuses. You don't want to connect the neutral and ground together anyplace downstream from the service because then you would be paralleling the neutral current. Paralleling the neutral would cause current to flow throughout the metal of the building, the plumbing, as well as the grounding conductor, creating galvanic corrosion and life safety concerns in the event that the actual neutral conductor comes loose and all the return current is flowing through the metal enclosure, plumbing pipes, building steel, and grounding conductor. If someone were to be holding on to two pieces of a newly opened section of pipe, they could be killed if that pipe were relied upon to carry the brunt of the neutral curent. The same would apply to any grounding conductor, or section of building steel, of section of conduit enclosing conductors.
