Yeah I understand where you are coming from, a family on my road have a fabric one that they put on their dog if he is out on the street, just to be safe whilst there are children around.
My dog has been attacked by another dog which was not on a lead. I think that is a big issue as well, I walk my dog several times a day in the park near where i live and the amount of people that walk their dogs off the lead is not right. Some of the dogs that do be off the lead would be dogs I would class as dangerous dogs; pit bulls, stafs etc, these to me are dogs which should be on a lead as well as muzzled at all times whilst in public places.
However, I suppose if your dog is well trained and looked after and is a real pet and placid and is always walked on the lead then maybe a muzzle is not necessary but in my opinion that depends on the breed of the dog too.