I think there was many factors that played into the collective thinking on prohibiting this practice. Probably one of the most significant issues that lead to the change was the requirements concerning identifying or grouping grounded (neutral) conductors to allow ease of identifying every conductor of a particular circuit.
Also, when grounded conductors are grouped together in such a manner, there is the possibility that if the connection back to the panel fails, all the circuits upstream from that point could potentially experience voltages at or near the line to line rating (ie 240 volts in residential).
Often, when an electrician or service technician needs to modify or alter the wiring, he or she must know which grounded conductor serves which circuits BEFORE breaking any connections. I once worked for a contractor where a service tech destroyed several computers in an office when he inadvertently broke the grounded conductor in a multiwire branch circuit and put 208 across numerous circuits!