These USB adapters have their own GPU, not a very fast one though.
The reason why these solutions won't support flawless video playback or a non choppy desktop behaviour is due to the limitations of the USB interface.
USB 2.0 has a maximum throughput of 480Mbit/s. Single link DVI already makes 3.96Gbit/s.
USB 3.0 would be more than sufficient for this (4.8Gbit/s) so we might see very capable USB solutions for additional displays in the future (at least for displays <= 1920x1200).
What is an average bitrate of 1080p movie? I guess it's around 10Mbit/s? HDMI has maximum of 10.2Gbit/s so USB is nowhere near real display connectors.
I really hope LightPeak is the future. It could be used everywhere! In displays, HDs, speakers etc...