I guess I don't really see the benefit of "true" virtualization, other than it's usability improvements. I generally want to squeeze out ever bit of performance out of the GPU if I care to share it with the guest at all (at least on my home machine). I'd be using it for playing games.

For the cloud, I could imagine wanting vGPUs so you can shard the massive GPUs that are used there. But in cloud, you would then have a single device be multi-tenant, which is a bit spicy security wise. Passthrough has a very straightforward security model.