The help text in the Field Mapping fieldset in the config settings implies that a service provider has already tried to harvest metadata and could not:

"Caution:The Service Provider could not retrieve your datas when you dont use the standard mapping. The blank fields do not appear on the OAI-PMH response."

This should probably be reworded "Caution: Service Providers may not be able to retrieve your metadata if you do not use the standard mapping. Blank fields do not appear in the OAI-PMH response."