When /etc/portage/make.conf has either of VIDEO_CARDS=i915 or VIDEO_CARDS=i965,
x11-base/xorg-drivers should pull in x11-drivers/xf86-video-intel
This way, the same VIDEO_CARDS= value will apply for x11-base/xorg-drivers as it does for media-libs/mesa.
And it would cause much less confusion for noobs not having to put two values into VIDEO_CARDS=
i915 and i965 are only flags for mesa, in order to allow more fine-grained control over which drivers to build. They are meant to be specified in addition to intel, not as a replacement.
The one USE flag to control support for _ALL_ Intel GPUs is VIDEO_CARDS=intel. Some ebuilds offer finer grained support, such as mesa.
IMHO, if we were to "clean up" Intel-related USE flags, I would remove all USE flags and just keep "intel". But right now, we have no such plan.
VIDEO_CARDS=intel is the way to go
*** This bug has been marked as a duplicate of bug 431794 ***