From an engineering perspective I can see a logical reason for the requirement. If a short circuit happens, say on the 15A circuit, and the #14 grounding conductors are carrying that fault current back to the panel then there is usually a pretty fair amount of voltage drop happening as it flows due to the high current. This voltage drop would be measurable between the #14 grounding conductors that are transmitting the fault current and the #12 grounding conductors that are not and both sets are inside the subject junction box. This could result in some arching (small as it may be) between the grounding conductors (or metal parts) as the potential on the grounding circuit is looking for any other path possible to get back to the panel. By bonding them all together, this potential is eliminated and the fault current path is improved which aids in clearing the fault sooner by allowing a higher current flow for tripping the breaker on short circuit. My 2 cents FWIW.