Currently tar without --keep-old-files silently overwrites existing files with versions from the archive. With --keep-old-files existing files can be retained but tar complains about each and every file. This can cause grief in postinst scripts, for example. It would be very nice to include a switch (say --dont-warn-about-existing-old-files) to avoid this.
However I do not see the behavior you describe on previous (Wheezy) versions of tar: --keep-old-files was silent before Jessie; see bug#784362.