You see its horses for courses. I spent a lot of time training dogs to do it for real. I have one here now and when someone knocks on the door he doesn't bark, create or try to take the door of its hinges but observes. I also KNOW that this dog can do the job for real as I spent several years training it. He is calm, obedient, stable with people and animals and to me this is what a guard/protection dog should be, calm and sociable in todays climate, but thats just my perception.
P.S. Many dogs will bite if "needed" and gievn enough stimulation, its what happens after that.............