Interesting point, but as long as they are declared to the underwriters - they are no different from crashbungs or pannier frames - something which can reduce the cost of a low-speed slider, but in a more significant "off" - can indent the frame, and result in the requirement for a re-frame which in all probability make the bike an insurance "write off". That same "off" without bunks might only result in plastics damage - who knows?
Al