Robin, "Useless" is perhaps a little wrong. "Worse than useless" in the hands of someone incorrectly trained in its use as it will lead to an unjustified level of comfort and perhaps reduced lookout. "Useful" in the hands of someone who understands it's limitations and uses it to assist their lookout/SA. With correct training, anything that can improve your personal safety is useful!
(PS isn't there some banter about the CAA forcing all bug smashers to have transponders fitted shortly? Can't remember and have been away from the UK soooo long I'm entirely out of the loop!)