This just opens up the system for abuse... and feedback.
Users could pretend to be angry to get a higher priority in the queue or to try and jump the queue.
Also, if the system records responses and tries to rate and categorise them it could be possible to play the responses back selectively for a human to determine what is and is not anger.
This is much the reason why I currently snarl into stupid voice menu systems.. "Do you have a client id number"? "NO!!!!!!!" "Please say your user name" "GRRRRRRRR".
Although, I have yet to have anyone ring me back about any of this...