Plus it's nice when systems syncronize. Gnome and X both have a six-eight month release cycle
and so do lots of other big software projects.
Application developers depend on Gnome's new stuff to get the latest and greatest features.
Gnome depends on features being present in X to get the newer features (like compositing or
xrandr improvements). X depends on the kernel for some of it's features (like DRM texture
management improvements for compositing and hotplug input devices support).
So by having everything sync up it makes Linux more reliable platform for software developers
since they can depend on release timelines. Projects have realistic expectations on new
features and users get the best improvements, better hardware support, and quicker bug fixes.
So on and so forth.
Another example is Gstreamer. With no time line for releases projects based on Gstreamer were
very hard for users to test and use since no distro had the up-to-date dependancies for the
software. In order to get some of the neater gstreamer-based applications working users and
testers would end up having to break dozens of other programs by compiling custom versions of
With Debian, for previous releases, people were very hurt because they wanted to use Debian as
a platform for their own projects. As months and years went by before another stable release
those projects died on the vine.
Six months is a good target to aim for. If you overshoot it then you'll get a new release in
eight months. As it works out (with holidays and such) you end up getting a rotating schedual
of 2 releases at the beginning and end of one year, and summer of the next. That's pretty good
and will help keep Linux very competative with Vista or OS X.