well, why do you think so? Personally I have not seen any changes in his behavior.

I suggest these devices are useful to signal to other traffic members much before time when you have to made decision about your actions. For example in this movie I've not seen moments, in which this device is useful. If you had not used that, would nothing have changed.
This device is better, if car (another bike, bus, ship, ocean liner, Statue of Liberty) in more than 10 meters from you. And if you prefer to turn and ride around this car, why do you need it?