Coming from the same direction as the inflatable roof rack is the automobile backpack. Not, not a backpack for humans that looks like a car, but a backpack that attaches the the hatch of your small car. Think about a VW Golf or a Honda Fit for a minute. Over the rear window, imagine an oversize backpack and you've got the general idea. Of course, not being able to see out the back window is a downer, but for the occasional times where a person might find their small car lacking in storage space, this roll around, waterproof pack might be just the ticket. If you're interested, check out their website here.
The benefit to this system is that when not in use, it adds next to no weight when you are carrying it around in your vehicle. Even when it is in use, the aerodynamic drag is less harmful than a roof rack. Maybe automakers should just start bundling something like this with small hatchback's the same as they bundle fitted luggage with their luxury cars.