A .res file is probably indication of some older tool in the past losing a file association, or trying to pack a unknown file into the hak. As others have explained, it has no meaning and is not used in game logic anywhere.
nwsync_write: Adding a switch to ignore them with a warning sounds fine to me, but it should be off by default. Data consistency is important and just packing up random unknown stuff for minor convenience is worse in the long run.
For this case: Instead of just deleting the .res, it’d be worthwhile to look in what it actually contains, because one has to assume it used to be something else in the past. This could be indicative of orphaned data, or some other problem worth looking into.