Hello resqcapt19
I will try to explain...as you know the main service neutral ("grounded conductor" as the nec like's to call it) is bonded to ground at the main service. This is the only place the code lets you do this, unless of course you have a separately derived system such as a generator. Now if this generator is using this neutral without a X0 bond at the generator (and a 3p xfer ) any leaks to ground or faults to ground will return to the neutral at the main service bonding jumper ("backfeed" in electrician talk). This is were it could be a safety issue, if someone was working on this neutral and open it ( this is something some people only do once.. they don't get a 2nd chance). A 4p xfer switch with its own X0 bond at its neutral, would be isolated from the main service neutral and could not backfeed when the generator stated.
Hope this helps... Maybe someone smarter then me may explain it better.
