Nothing wrong in my book with using brakes as a signaling device. I've got disc brakes on my commuter, and if I put just the right pressure on the front set I get a tone out of it. Sort of like running one's finger around the top of a water filled glass; though, not as pleasant a sound.
Where the brake noise helps is when I come around a blind corner to the spot under a bridge where indigents hang out while waiting their turn to panhandle the nearby car offramp. If I do it right, I get a good tone as I come around the corner and I can skip yelling at them to clear the path. The last thing I want is to have a bunch of angry homeless types on a vandetta against me or some other unsuspecting cyclist.