IMO, the reason why Apple didn't place a USB port directly into the iPad is because if they did so they'd potentially have to support every type of device that could attach to that port. They didn't want to do that so they offer a "camera connection kit" that is warranted to work only for the transfer of images from a camera. If it had a standard USB port everyone would be wanting to connect their keyboards, mice, printers, hard drives, video cameras, etc. and if those didn't work there would then be a loud outcry asking why not. Thus, by not providing those ports Apple keeps the system closed and avoids all kinds of compatibility and device support issues.
Furthermore, if they allowed the attachment of just any type of read/write storage over USB or even through a built-in SD card slot then there would be all kinds of issues about file systems and questions as to why you couldn't use the external disk or SD card for application storage or any other type of data that you might like to offload from the internal flash memory. Note, in any case, that the iPad/iPhone has a file system structure that is sandboxed and quite different in layout from the desktop versions of Mac OS X or Windows.
Sure, having a built-in USB port and/or flash card slot would expand the range of options that could be offered on the iPad but it would also mean that Apple would have to support those options and they probably don't want to do that (for a variety of reasons).