Quoting platypus: "I've had low-speed offs wearing two different HG textile jackets, and both have been shredded." Bear has alluded to impact protectors that can flap around in loose-fitting garments, and consequently be out of position when the wearer hits the road. You have identified a different problem: what if the structure of the garment is so weak that it fails catastrophically on impact? The protectors might be in position for the initial impact, but as the garment falls apart around you, there is a risk that the protectors will join forces with the tarmac to work against the fabric and speed up the destruction process still further, whilst simultaneously being out of position and unable to provide protection in subsequent, albeit possibly-lesser severity impacts. All very theoretical, but I believe the premise is sound.