I completely agree. When I was growing up we had a lab/shepherd cross; she was extremely smart and friendly, but also very protective. I'm not sure how my parents trained her but I don't believe she was actually trained to be protective, it just came with the dog.Leaving a dog to judge what constitutes a threat, and act upon that judgement, is leaving a HUGE liability in your hands and is a disaster waiting to happen.
When my uncle walked in unannounced when we weren't home she attacked him. She would have done a lot of damage if he wasn't wearing a jean jacket.
It is not a position that a dog should be in in my opinion. There are too many things that could go wrong, and the dog will always be the one that has to suffer because of it. Nothing happened with that dog after that incident because it was family. We were quite lucky with it. But luck runs out.