I have a similar setup (external monitor with the Macbook lid closed) But in my case (and possibly for other Macbook users with the same setup), closing the lid makes the webcam inaccessible as well.
Maybe if I primarily used a laptop directly, I'd have a different opinion, but having played with alternative input devices (e.g. Bluetooth remote control), I really like having a unified method of muting all my media, and in a familiar form factor. Since holding my hand up seems very natural and familiar to me, I imagine that I'd like this hand-recognition system.