I'm imagining a situation where you need to run windows on the host, but you do not want windows itself arbitrating network access -- so you pass through the real NIC to the vm, and route through the vm with a virtio (I think TAP is actually the only option host-side though but still) NIC.

Ah, I see. Given that you can even pass disks, gpu's, usb devices etc via pci, I would always choose the opposite route: run windows in a vm.