Here's a question for you dog owners/trainers. How do these dogs react to invited guests in the house? Can you welcome someone, even small children outside the family into the house without worrying about the dog attacking? What if that person stays the night in the house, can they not leave their room during the night over risk of getting attacked? I ask because I have no experience with well trained dogs.
I speak only for myself.
I would never keep for a pet a dog that was being trained for K-9 work as they are, for the most part, very "hard" dogs. By this I mean that they are trained to be as tough as possible, not necessarily overly aggressive, but ballsy enough to handle the job that they are tasked for which leads them to ultimately not "play well with others". It is a shame though, as most are hard to place in homes once they "retire" because of how highly driven they are.
I prefer my dogs well balanced. Training in Schutzhund work is for the trials. The Schutzhund trails are a breed suitability test for the German Shepherd Dog. It's original use was to see if the dog displayed the appropriate traits and characteristics of a proper working German Shepherd Dog such as: trainability, high drive or desire to work, courage, protective instinct, intelligence and a strong bond with the handler. Most dogs that pass the Schutzhund trials are suitable for search/rescue work and/or police work and are suitable for breeding the next generation of working dogs.
I do not have any desire for unsolicited aggressiveness in my dogs. I can take my dog anywhere, sans leash, without fear of it hurting anyone or leaving my side. I can welcome anyone that I please into my house to visit or spend the night and my dog is just fine with them, especially children. The only time I ever had an issue is when we had guests with a child that was misbehaving and (deservedly) got a spanking. The dog insinuated itself between the parents and the child, yet was still not aggressive. She barked once and lay down between the child and parents. It was almost comical.
My dogs have never been animal aggressive, either. If you train a dog to be confident in it's abilities and strengths, most of the time they are completely calm because they know that they are able to handle any situation required of them. While all of my neighbors' dogs are raising holy hell and carrying on, mine is as quiet as a mouse.
A well balanced and confident dog will "know" when there is something amiss. They can hear, smell and sense things that we can only read about. My dogs have always "worked" for me because of their love to do so. It is their nature and in their genetics. I have never had to be mean towards, punish my dogs or force them to learn what I require of them. They do it because I develop and focus their natural drives.
The idea of a protection dog is not to make them as mean as possible (which usually results in them being unpredictable) but to make them balanced and confident that they can protect you and yours.
Again, anyone can do it. Even a 7 year old girl
(notice how aloof the dog is to all of the distractions and how it alerts to the agitator but does not bite until he moves)
http://www.youtube.com/watch?v=Sn3VghnNEpc