Maybe useful only for combination with %config(noreplace), not sure at the moment.
There are several files e.g. Fedora's /etc/passwd marked %config(noreplace) where it doesn't make sense to .rpmnew file. It is useless and could confuse users (and even cause troubles if used as replacement for /etc/passwd by unexperienced admin. Therefore I would like to not create .rpmnew file, to not replace old /etc/passwd file but to INSTALL /etc/passwd file when installing rpm's.
There is afaik no option for this case available at the moment. When I mark the file %ghost, it will be not installed - as it will be not part of .rpm file - which is not good in this case. Deleting .rpmnew file in %post could make paranoic user scared - as rpm update reported .rpmnew file and file is missing. If you have other proposal, feel free to post it here.
Could be useful for some catalog files - which have to be installed once, but there is no need to update them afterwards - even if the file content in new release changed. I do not expect wide usage, but it would be an option for such border cases.