I haven't, and I don't think I ever would.
I like my horses to hear everything that is going on around them and still listen and pay attention to me. I need him to get used to all the sounds around because it can help with all the other unexpected noises on the trail or a quite ride alone, etc.
At this last fair when I showed my horse, Brisco, he was one of the best horses out there. (I'm not trying to brag, but he was.)
There were rodeo cows being unloaded just a few yards away from the arena, A car alarm went off suddenly in one of the quietest times, and the wind was making some of the horses nervous with all the sounds around just moving with the wind.
Almost all the horses got away from their handlers when the car alarm went off - it went off and three horses darted forward and started freaking out. A few behind me freaked out still, but not as bad. And I am not kidding you, my horse was asleep the entire showing time, I'm pretty sure he was sleeping when I was riding him too!
But my horse didn't care about anything that was going on around him, because I have taken him plenty of places and and done many things with him that involve sudden loud sounds, sounds right next to him, sounds far away, etc.
So unless like was Incitatus32 said with the horse needing some, then I would say to not use them. But that is just my opinion.