I have had my Liberty for only six months now, but my impression is that:
a) People who want to add a subwoofer generally are audiophiles who want something better than the OEM one
b) People who are happy with OEM stuff generally wouldn't bother with a subwoofer at all
c) The main advantage is the custom-fit box it comes in which makes for a cleaner install. That's not enough to sway the audiophiles who will put up with all sorts of modifications in order to get better sound, and not enough of an improvement to make those who care more about custom fit than sound to bother.
To be honest and fair, I've never personally heard
a system with the OEM subwoofer, so this is based solely on secondhand comments I have come across while researching the subject myself.
(Ain't much help, am I?)