Yes, if it could actually be made to work accurately (down to the few-pixel level) and in real time then it would be a brilliant user interface element. Why sho

Yes, if it could actually be made to work accurately (down to the few-pixel level) and in real time then it would be a brilliant user interface element. Why should I click a mouse when I can just stare at the "reply" button for half a second? (Yes, all sorts of potential badness which could happen if implemented unwisely.)