Let me start by saying that owning a good PPD IS NOT A LIABILITY. If your training is sound, and you are very responsible there is no liability. No more than owning any protective dog. In fact dogs trained in protection work are less likely to bite someone because they are trained to respond apropriately in different situations. I feel much more comfortable around PPDs than I do around untrained protective breeds that don't understand human aggression and don't know how to respond in most cases.
Now, I have personal protection dogs. I have owned them for around 10 years.
There are a lot of reasons, one reason I own them is security. In my business I don't make a lot of friends, in fact I have received three death threats in the last 5 years. I work long hours, family is home alone often, so I have created my own little fort complete with protection dogs, security system, and physical barriers. I consider what limited liability (I don't even like saying liability because that only applies if you have a crap trained dog and are a crap head that can't keep his dogs under control or contained.) there is involved in owning a trained dog a minor negative to a long list of major positives.
Another reason is because I have a family history in GSDs. Going back about 85 years we have owned and trained GSDs in some capacity or other. The GSD is a working dog, we train and breed and own them to work.
Yet another reason is once I started learning about protection training and police service training I GOT THE BUG! It is fun. I train for enjoyment. I always enjoyed sport, but honestly wasn't very good at it to start with. I seem to have a nack for protection work and picking dogs. My protection training is an extension of Schutzhund because I title my dogs and then move on in protection training. I have since returned to sport training because it presents a challenge.
Did I mention this one yet. THE GERMAN SHEPHERD DOG IS A WORKING DOG, PROTECTION IS GOOD WORK. YOU SHOULDN'T OWN A GSD UNLESS YOU WORK HIM IN MY OPINION.
Even if you don't have a job where you irk people, there is a good reason to have a trained dog. You never know what is going to happen out there.
Do you consider PPDs the same as sport dogs? I think the liability is probably exactly the same for people that train for Schutzhund or French Ring as it is for people that train (legitimately and correctly) for PP. A court isn't going to make a distinction I don't think. Both are bite training.
I also think protection training is important for breeders of working dogs. Sport and personal protection and police service training allow you to judge your dogs and breed solid capable working animals. If it wasn't for Schutzhund the working GSD would be long gone.
There is nothing wrong with protection training if you are doing it right. If you are a ghetto, redneck, gang banger, back woods, homie, white trash, drug dealing, drug shooting, underbelly of society, criminal out training crack dogs to guard your stash and fight in the pitt you are a jackass to begin with and should be in jail or dead. Those are the types that give protecton training a bad name. I am starting to see more of those types entering into the legitimate sports. That is a sad state of affairs.
I don't consider well trained personal protection dogs a liabiliy. If someone attacks you, breaks into your house, or goes after your kids, I would like to see them try to take me to court. The legal system isn't that bad. You just have to know your laws and be careful. My dogs will NEVER accidentally hurt anyone.