Honestly, I agree with that the usefulness would be great since I could gtalk/hangout or even use Skype depending on compatibility. But from experience and availability, it looks like it's not really A feature that companies seem to have high on their lists. I have tried even plugging in a USB camera to mine (Sony Google TV unit with up to 3.2) and it does not work. I had heard "once upon a time" that the Logitech units could do a basic video calling to other sister units before one of the software updates, but then the feature was removed.
It does feel like some companies would rather release a device that directly connects to a TV with its own software and higher price (example: Logitech has a device that does just that, but costs about $200) than make something that could be used for those of us with a Google TV and would be a lower price point.