For years I have used my laptops in clamshell mode, and for years I have been using the BookArc. When your laptop is in clamshell mode it doesn't vent and stay cool as well as when the lid is open. However, with the BookArc, the underside of your laptop is exposed to the air and so it helps keep the machine cool when in use.

Moreover, the BookArc helps with cable management and it uses a smaller footprint than if the laptop were set down flat. Now that I have a MacBook Air I have a the Air-specific BookArc and it fits perfectly. In my opinion, Apple should bundle a BookArc with every Thunderbolt Display.