According to http://nationbuilder.com/overwrite_existing_data "Existing data cannot be cleared by importing empty fields." Essentially, an empty field in an import is left unchanged by the import, I'm not suggesting changing that. What I would suggest is that a special value in the import is treated as "clear field", I would suggest the ASCII value DEL. My use case is an application that imports large amounts of data via the API. When it needs to clear the content of a field it uses the People Update Endpoint, however this is much slower than using import when large amounts of data are involved.
Hey Andrew, thanks for your suggestion. We do have the option to clear fields (point person or home phone number) by batch updating a list. However, I do understand how this could be helpful when updating data through the importer. We don't have a timeline for this but we will keep you updated.