Good question JD, although its hard for me (engineer and attorney) to keep it brief I will try my best lol.......If you have a basic understanding of electriciy this may answer your question, but if not it would take me a lot more words. NOTE before Billy Bob has a calf, this is NOT perfectly detailed and complete and perfect and all 100% correct, its to give a lay person a basic answer only. The whole story and 100% accuracy would take a ton of explaining
The Neutral GrounDED Conductor is a hot live current carrying conductor, thats why its insulated so you dont touch it and it doesnt touch other things which you might touch.......If you touch it you are placing your body in paralell with a live current carrier and maybe theres a path from your hand to the ground or the other hand back to the panels ground etc AND CURRENT THROUGH THE OLD TICKER CAN KILL YOU DEAD.
The Equipment GroundING Conductor is often a bare wire, no harm in touching it, its NOT a hot live current carrier like the insulated Neutral. In fact, its attached to the the outer conductive metal case/frames of tools and kitchen appliances WHICH YOU TOUCH EVEN OUTSIDE ON WET EARTH!!!!!!!! Its ONLY purpose is to return FAULT CURRENT.
You wouldnt strip the insulation off the Neutral and let your grandkids touch it would you?? But you do let your grandkids pick up and touch say a kitchen appliance or one of your tools that are plugged in right???
SO IN A NUTSHELL if you start mixin n matchin or substituting or re bonding Neutrals to equipment Grounds ANY PLACE DOWNSTREAM AND OTHER THEN THE MAIN SERVICE ENTRANCE BOND the equipment Ground then is in paralell with Neutral and becomes a hot live current carrier same as the INSULATED NEUTRAL and when you touch the case of a tool or appliance YOURE TOUCHING THE SAME AS THOSE INSULATED NEUTRALS and placing your body in a possible current path.
That and all the other single point grounding theory and avoidance of ground loops is why you DO NOT re bond Neutral and Equipment Ground busses in sub panels.
AGAIN THIS IS NOTTTTTTTTTT ALL PERFECT AND 100% CORRECT AND NOT THE WHOLE ANSWER but I have to run to town to meet a buddy for lunch and the best I can do for now. Hopefully it gives you a start at least to understand why the Neutrals and Grounds SHOULD NOT BE REBONDED AT SUB PANELS. Neutral is hot insulated live current carrier while ground is a bare conductor intended for fault current ONLY and its atatched to metal frames of things you touch. Do you really wanna mix n match n sub one for the other and let the grandkids touch those hot current carriers barefooted standing on wet ground??? Im NOT saying thats necessarily the case, but given the right set of wrong circumstances (faults, breaks in lines, shorts etc) that can happen and why you SHOULD NOT re bond Neutral and Ground in sub panels
Got it????? I will check back later today, gotta run for now, maybe the other sparkies an add to this but as always BEWARE of untrained Billy Bobs who say its fine if you do!!!