well, i just simulated a runaway X at nice -19 on CFS (on a UP box), and while the box was a tad laggy, i was able to killall it without problems, within 2 seconds that also included a 'su'. So it's not an issue in CFS, it can be turned off, and because every distro has another way to renice Xorg, this is a convenience hack until Xorg standardizes it into some xorg.conf field. (It also makes sure that X isnt preempted by other userspace stuff while it does timing-sensitive operations like setting the video modes up or switching video modes, etc.)