Maybe you you should reexamine your needs.I would like to be able to use my m-audio box to input, for example, my guitar. It works by USB.
To use the iPad, you need a host computer. Maybe that's what you really should be using for such applications, not the iPad.
To elaborate: to do so, you now expect Apple to not only add space & weight & cost for physical USB, they have to provide the Pandora's Box of allowing/supporting installing & updating 3rd-party USB device drivers - a manifestly complex process which goes FAR outside the paradigm of the iPad (pure wireless, pure information, nothing wired, no low-level data transport concerns).