Honestly, I don't expect much out of something that was an afterthought. They 'may' have initially thought about adding USB functionality but maybe they ran into issues (maybe speed?) and decided against continuing with it.
I personally would expect no more than casual printing or USB key file sharing via any wireless router USB port. It's a convenience feature and really nothing else. If you really want data storage then get a server hooked up via GigE, or a lower end "NAS" box hooked up via GigE, and NOT USB. If you're using external enclosures for any serious transfers then you should be connecting with eSATA and not USB.