I do physical layer testing of ethernet switches. I want to virtualize the test servers for scalability, reliability, and a VM solution allows easy change of system configuration.
The bottom line is that testing often requires opening the network port in promiscuous mode to sniff traffic, many tests need to be able to use ethtool to view network properties (speed, duplex, link state, etc) and to set properties (auto-neg, forced speed, duplex, link state, etc).
We use TOE cards which also allow offloading data from the CPU to the card allowing higher throughput, some tests we use this to send ~1Gbps traffic and monitor the data rate.
So there are multiple scenarios, the main one being the ability to see and set physical layer properties, the other being able to offload traffic for maximum throughput. The latter can possibly be accomplished through OS layer drivers on the hypervisor but the former requires better driver access than what i currently see in the VM.