Not all dogs trained for protection go to live in a junk yard or walk around prison walls. Some people train their dogs for the sport of Schutzhund or just for family protection. I did it just for fun. My dogs loved it and it did not change their attitude toward strangers at all.
I started a few classes in protection training mostly because I wanted to get a Temperament Test Title on my dogs. This title requires that the dog pass a series of tests, including being friendly to a stranger who greets them in a friendly manner, to pass someone who is neutral, to walk on strange surfaces, to be startled, but not terrified, by something like an umbrella or a loud noise and to act protectively if a stranger comes toward them looking hostile. I knew my dogs could pass the first part of the test, but had no idea what they would do with a hostile stranger. I thought I would take them to a class and see.
I learned that a dog that is well balanced and socialized does not change his personality if he learns to protect.. A dog who is fearful or overly aggressive is not a good candidate for training. It would just increase the likelihood of unpredictable behavior