MrChu, I also tend to have more AGR points than I can use on Amtrak itself. Another good use is to transfer to HHonors--I've done that quite a bit. In fact I only started using the CO transfer function this year after the CO-UA merger (I never fly CO, but started an account there once I could shift credit-card- or otherwise-non-flying-generated points to UA).
I am also hoping that S+ members will eventually get to use Red Carpet Clubs. I have no idea where the CO lounges are since I never fly them!