I really don't like it...or the idea of it.
My grandma uses one for her dog. If there is no reason other than the owner of the dog not wanting to hear it, I think it is completely unnecessary.
My grandma has one for her dog that she uses when she has company over because he has a tendency to get extremely noisy when new people come in. She lives in an apartment, and if her dog causes too much noise, people get calls. I especially dislike it because her dog definitely acts differently when he gets the collar put on. He immediately gets quiet and shy, which is really weird for him because he's normally an in-your-face outgoing dog.
Silencing your dog just seems absurd to me. It's like silencing a child.
So I really don't like it.
But hey, it's up to the owner.