As Needa mentioned, the edits to a NEF are done to a side card file, not to the original RAW file. If you don't save the RAW file, the side card file is useless. If you convert your NEF's to DNG files, then the edits are applied directly to the DNG and not to a side card file. In that case, you could opt to not save the original NEF. BUT if you ever enter an image into a photo contest, sometimes they require the original file.