I also fail to see what you gain from the middle rings. The discussion is mostly what should be in ring 0 / kernel space and what should be in ring 123 /userland.
Another problem is that people think that moving something to ring 0 improves performance (anybody remember the HTTPD module?), but in reality, this is not always so.
But I am no expert in OS design; I regret that HURD is becoming increasingly irrelevant, as it has some interesting ideas. Besides, I'm spoiled: I dislike having to reboot just because I have upgraded my kernel
