Steve, with reference to ordinary friction sway bars, they greatly reduce the trailer coupler's freedom to twist on uneven surfaces. So that is why I remove the sway bar on rough forest roads -- to let the trailer move independently of the tow vehicle.
Often, in really bad conditions, the tow vehicle is tilting to the left while the trailer is tilting to the right -- that does not happen on pavement! So my goal is to let the trailer tilt as much as it wants to, thus minimizing torque on the whole hitch ball and coupler assembly.
I'm not sure if the same reasoning applies to a WDH -- I don't know enough about WDH systems to comment.