>> One nice thing about using SMB/CIFS would be that it could be implemented for most current operating systems with a simple USB-to-IP proxy and existing network filesystem drivers.
> You could even do it with no added PC software -- have your phone be a USB RNDIS interface that runs a DHCP server, and run a SMB/CIFS fileserver with all the right discovery stuff that makes it just show up in your normal network browser.
That assumes your PC is set up to automatically establish links over arbitrary USB network adapters, which seems like a rather insecure default. The USB-to-IP proxy in my proposal would only allow the device to present a restricted SMB host on a private, non-routable address range; it wouldn't be able to choose its address or access other services on the host PC.
Even ignoring the security aspects, I believe desktop Linux setups, at least with NetworkManager, normally prefer wired network connections over WiFi, and I wouldn't want my WiFi-connected laptop to disconnect from the Internet every time I plug in my tablet or smartphone.
IIRC, on Windows it chooses the faster network as the default route, unless you configure the metrics manually; depending on speed of the main network, the result may depend on whether the device shows up as a 100 Mbps or 1000 Mbps adapter. In the latter case (necessary to max out the USB at 480 Mbps) it could even cause issues for 100 Mbps LANs, which are still common, much less WiFi at anywhere from 12 to 300 Mbps.