There are disadvantages to the concept too, such as the unit requiring a network connection in order to function, whether it be wired, wireless or even cellular. Also, it pretty much precludes the laptop for any work except what the remote server offers. However, HP thinks their Compaq 6720t unit has many selling features over other mobile thin clients, such as a full array of expansion slots. A thin client with the ability to attach USB devices, flash storage, PCMCIA cards, et cetera, definitely has an edge over something more plain.
It's definitely an interesting concept, though is obviously aimed primarily at businesses or other institutions that have a need for such a thing.