I would expect a pro to be carrying adequate memory cards and not need to be transferring DURING a shoot.
But I'm barking up the wrong tree anyway - Harry's scenario is transferring from camera to ipad to view the pictures on the fly. Having a hard drive as an intermediate storage would be a pain in the neck for that use case.
Then again, I'm still of the opinion (to stay on topic...) that the ipad is NOT at risk for losing ports, but the iphone IS.
(Here's a nice thought though - a
hard drive with an SD slot built in)