When you 'drop' a datafile, it is not removed from the data dictionary. The key issue is that the file is still referenced in sys.fet$ (free extent table). When a segment attempts to allocate a next extent, it may try to use the datafile that no longer exists. This will cause the invoking transaction to fail. By forcing oracle to skip over the file (as per the metalink article), you insure that this situation will never be encountered.

If you do this, make certain to document the reasons. Better yet, take the next opportunity to drop and recreate the tablespace. Of course, if it is the system tablespace, you are going to have to drop and recreate the entire database.