There isn't one. It's an existing option in the kernel, you can configure cgroups that way already, but most people don't do this so the feature is wasted. All this patch does is roughly approximate a decent-looking cgroup configuration by splitting processes by tty automatically.
Of course there will be a regression if you change the scheduler policy. ck tried something similar to this and mplayer performance suffered with it (though I don't remember the details). It also broke gnome-startup because it assumed some specific schedule ordering, though this patch is more limited so it might not.
I got upvoted because I'm right. Kernels don't need to support horribly designed programs just because they exist, just like they don't need to support horribly designed programs that don't exist yet. Kernels support an interface and that's it. If you write code that abuses the interface, get ready to become a regression, and that'll be your own fault.
(TBH I have no idea why I got upvoted, it wasn't that insightful, but I stick by what I said)
(EDIT: I'm talking about gnome-startup. That's a stupid regression that never should've happened. The mplayer performance bug is totally understandable if you're mucking with the scheduler. What we really need is for someone (distros?) to pick up cgroups and provide a nice UI for it, some sane but nondestructive defaults, etc. Until then, this is a nice patch that keeps badly behaving programs from dragging down the entire system. At the very least, we mostly get user separation in multi-user environments.)
