Do you honestly see yourself using your iPhone as a USB drive? Even if it had that functionality (it doesn't), how would you plug it in? You would have to bring the cable with you, which is far less convenient than a USB thumb drive.
How different is bringing a coiled up small cable along than a thumb drive?
Bringing a cable along with you actually makes more sense because you can use it to charge your phone with any computer on the go as well as use the phone like a USB drive.