From the UI Interest group (July 22), we discussed the use cases in which people would want to make "skeleton" metadata-only objects even though the digitized files weren't available yet. However, it's been customary to make the file upload mandatory.
Fix would require going through all modules' includes/[file]_upload.form.inc and removing the "#required" => TRUE line from the form definition, then checking whether a file is present in the submit handler.
This functionality already exists in the batch uploader, i.e. you can bulk-create objects from only XML files, so that's a way to have the same effect. But it's somewhat clumsy because you have to create and save the XML to disk first.
I'm curious to know the reasoning for which the file upload was originally made mandatory, if that can shed some light on whether we should do this.