I'm not sure that USB support would open a lot more doors than Bluetooth; but it would open some different doors. Notably, whatever USB flavour is offered, it needs to support the isochronous transfer mode. That opens up the door to high quality digital audio with external devices; and, if the overall system is fast enough, it would open the door to webcams.
Bluetooth via a dongle would open up the Bluetooth door to far more people than Bluetooth via built in hardware special to the A9, which in turn would open up more sales to Ad6. Or so it seems to me, anyway.