I only use my hard cases if the ride is all paved or very easy forest roads. If there's any unpaved adventure anywhere in my ride, I always use soft bags for the obvious safety reasons. That being the case, I've never felt the need to protect the hard cases. They're pretty tough and I suspect would be fine in a parking lot speed drop. Any more energetic than that and you've got more to worry about than the condition of your hard cases.<joke alert>
But... the panniers are the crash bars.
<end joke alert>