The reason the code requires / permits the EGC's of different circuits to be connected together is due to the fact that EGC's serve as the primary effective fualt path for circuits.
With the EGC's of different circuits connectred together at junction boxes and other enclosures, a ground fault on the load side (downstream) of the box or at the utilization equipment will return all paths back to the source. With the fault current splitting among 2 or more EGC's, the total resistance of the effective fault path is reduced thus resulting in less stress on the insulated ungrounded conductor and reducing the trip time of the branch circuit overcurrent device.
The "other" persons argument makes no sense in that a metal junction box may contain several circuits that are brought to the box in metal raceways thus bonding all metal parts and EGC's together through mechanical connection. The same concept need not be changed for non-metallic wiring methods.
Be certain the person is refering to grounding conductors and not grounded conductors. Grounded conductors of separate circuits would NOT be permitted to be connected together.
