The advantage to the other company of making public the specs to the NIC would be the same as the advantage to nVidia - happier end users because of better OS support of the hardware.
As for hanging stuff off USB - the point of having a small case (like the Shuttle that's currently available with nForce chips) is in having the system be compact as possible. An extraneous external device defeats that goal. There is such an animal though (see http://www.tomshardware.com/network/20021122/
) but it only works in Windows (not DOS or Linux).
If you look at the NIC drivers included in the kernel code, it's pretty much everyone in the industry aside from nVidia (and these USB NICs). This doesn't seem to have hurt 3com's business, for instance, has it?