There is a note in http://fedoraproject.org/wiki/How_to_create_an_RPM_package#Other_tags, which states:
Do NOT use these tags
Packager
Vendor
Copyright
With no explanation given. Packager is quite commonly seen, so, when I am creating a new spec file to distribute software, why should I not include that tag (or any of the named tags)?
Copyright has been deprecated in 2000. Rpmbuild refuse to accept it since 2005. It was poorly named initially and should have been named License
from the very start. Nowadays everyone uses License
tag.
Vendor is usually set by build system (Koji, Copr, OBS) so you should not override it. I.e. every package in Fedora has Vendor set to "Fedora Project".
The same for Packager. But this one is a little bit relaxed. E.g., Koji set it, but Copr not.