because the only place where the ungrounded conductor and the ground can be bonded is in the main panel (service entrance).
That is not entirely true. There are a few instances where you may be allowed or required to bond the grounded and grounding conductors.
Separately derived systems for one.
Separate buildings or structures for another.
Check out the option allowed by 250.32(B)(2)
250.32(B)(2) Grounded Conductor. Where (1) an equipment grounding conductor is not run with the supply to the building or structure, (2) there are no continuous metallic paths bonded to the grounding system in both buildings or structures involved, and (3) ground-fault protection of equipment has not been installed on the common ac service, the grounded circuit conductor run with the supply to the building or structure shall be connected to the building or structure disconnecting means and to the grounding electrode(s) and shall be used for grounding or bonding of equipment, structures, or frames required to be grounded or bonded. The size of the grounded conductor shall not be smaller than the larger of
(1)That required by 220.22
(2)That required by 250.122
Any other location to panels downstream the neutrals and grounds have to be separated to avoid loosing a neutral and the ground picking it up and carrying it back to the main panel or creating another path.
The 'other path' problem is covered by this part of 250.32(B)(2)
there are no continuous metallic paths bonded to the grounding system in both buildings or structures involved,
I agree with Greg this has nothing to do with the possibility of loosing the neutral.
