Interesting. Were the mounting points for your replacement bar pristine, or were there any marks (gouges in the undercoating, etc.) to indicate that the car once had one (but that a previous owner removed it)?

The only (although admittedly highly unusual) reason to remove it would be to enhance the rear suspension's ability to traverse very uneven terrain -- e.g., a rough, off-road trail to a favorite cabin in the woods or fishing spot -- since the antisway bar tends to reduce side-to-side independence. Some very high-end SUV's that are used for off-road travel have systems to literally unhook the antisway bar's links at the push of a button (and then reattach these when they return to paved roads).

On the other hand, perhaps it was Volvo's decision to forego the rear antisway bar, especially if they felt that there wasn't enough understeer (with the existing front bar), which manufacturers believe is the safest default condition for their consumers.

I'll be watching subsequent responses -- a very curious issue.