Based on the above, perhaps the upstart script for samba / nmbd has a bug as the script will stop the system from booting if no net adapters come up. A preferred behaviour would be for nmbd to fail to start but let the machine boot.
Better still, if nmbd could have the same behaviour that smbd has, an that is to start and wait for a suitable adapter to come up.
Based on the above, perhaps the upstart script for samba / nmbd has a bug as the script will stop the system from booting if no net adapters come up. A preferred behaviour would be for nmbd to fail to start but let the machine boot.
Better still, if nmbd could have the same behaviour that smbd has, an that is to start and wait for a suitable adapter to come up.