When was the last time you actually pegged your CPU so the behaviour of the scheduler was important to you? Sure, some people who do render jobs or large compile jobs do this regularly, but I expect a growing amount of this to be done on dedicated hardware.
Personally I have a problem with disk I/O a lot more often. I suspect there is a significant amount of CPU work involved in SATA I/O and that this activity is not scheduled together with userspace, because when there is heavy disk usage interactivity in my terminals becomes very choppy.
This was actually better ten years ago, on yesterday's hardware, so something must have changed since then. If you want to help dekstop interactivity, test under disk activity first. Your users will thank you (at least I would).