Are they worthwhie?
I have had the chance to hit a cue that had one, and it does seriously reduce the feeling of vibration.
Imagine how a tuning fork works. You hit it, and it vibrates for several seconds. Obviously a cue doesn't vibrate that long, but the waves do move up and down the cue.
Now, imagine putting a rubber bumper at the ends of the tuning fork, and imagine how much the vibration would be cut down.
Steve