jhansonxi :
If power consumption is the only consideration then that would suffice but I suspect performance is more important for most people. I'm waiting for USB 3 devices.
Performance for what?
USB-to-HDMI/DVI/DP adapters are not intended to replace PCIe-based GPUs for panoramic gaming. People who need to look into USB video adapters usually need them mostly for 2D such as extra text or otherwise mostly static data/graphics displays. For those applications, USB2 is usually more than adequate.
With 2.5W maximum power on USB2, it isn't really possible to do more than a dumb frame buffer. Even with USB3's high-power profile, you still have less than 5W which still does not allow much of a GPU so USB3 models would most likely be dumb buffers as well.
If you want a "performance" USB-based display adapter (something with a real GPU), you would need something with its own power brick and that usually is a no-no for most people on-the-go - the fewer power bricks, the better.