The reason that CO has the Saturday stay restriction is that not enough people yelled when they instituted it several years back. I believe another airline (UA?) tried this and there was an outcry and they backed off. As a CO Platinum Elite, I am now paying for it. Still, it is still the best program.
There is and will continue to be inequalities between NW and CO. Remember that even though NW "owns" CO, they are supposed to opperate independently. There is probably a legal limit on how closely they can allign the programs. Also, Both programs developed over the years by providing benefits that each airlines marketing program felt its customers wanted. They will tread carefully in changing benefits.The inequality that irks me the most is the inability to upgrade Internationally on CO on the cheap fares. On NW, one can use miles to upgrade on ANY fare. I wish CO would change this.