It is almost impossible to describe, what the exact semantics of the USE flag "extras" is. Therefore, it does not make much sense to use it globally, because the effect is very heterogeneously spread over many packages:
app-admin/rsyslog:extras - Add support for the UDP spoofing module (omudpspoof) using net-libs/libnet
dev-python/PyFoam:extras - Enable optional dependencies
dev-vcs/subversion:extras - Install extra scripts (examples, tools, hooks)
games-board/pasang-emas:extras - Install some extra themes
games-puzzle/jag:extras - Install additional graphics
kde-base/kdesdk-misc:extras - Build po2xml and swappo tools
net-im/psi:extras - Enables extra non official patches
net-im/tkabber:extras - Enables extra non official patches
net-irc/znc:extras - Enable some additional modules
sci-mathematics/normaliz:extras - Install sci-mathematics/Macaulay2 and sci-mathematics/singular packages as shipped by upstream
sci-physics/bullet:extras - Build additional libraries
sys-cluster/glusterfs:extras - Install extra helper scripts
sys-fs/udev:extras - Compile udev-extras requiring external dependencies
www-apps/bugzilla:extras - Optional Perl modules
www-apps/ikiwiki:extras - Installs additional modules used by ikiwiki plugins
www-apps/postfixadmin:extras - Install contributed scripts and plugins
As far as I understand USE flags, they express cross-cutting aspects of a system, eg. strategies, policies, style, ...
I'd rather recommend to rename "extra"-s use in the various packages to something, which expresses in a reasonably abstract manner, what the strategy is, that is supported by use of this flags.
Eg. "bugzilla:extras" could be renamed "bugzilla:perl".
forked from http://bugs.gentoo.org/show_bug.cgi?id=349487.
Set Mike Gilbert cc:.
I'm not quite sure where to direct this bug.
I will say that "extras" is not technically a "global" use flag; it has no entry in /usr/portage/profiles/use.desc.
This type of discussion should take place on the gentoo-dev@ mailing list. Until that time, there is little to nothing we can do here, and when the time comes, this bug report might be reusable as a tracker bug for each package that is decided to be modified in this respect...
Btw, I've wondered about a couple of those instances of "extras", and it is probably as good as "contrib" would be as a USE flag. It doesn't say anything about the nature of the optional stuff you're enabling or disabling.