I think it's very difficult to judge whether an individual untrained dog will protect you unless it is in that situation. If we're talking just barking and maybe even biting someone who breaks in but then is backing off, then I think that most dogs will. However, I think the key comes if the aggressor fights back--a lot of untrained dogs will back off then.
One of mine, who is a total pet, I know would protect me because he has. He bit an ex of mine who tried to hit me during an argument, and wouldn't back off even when the guy was hitting him trying to get him off (and yes, that was the night the guy became an ex!). This dog is the nicest, friendliest dog you've ever met, too--he's the kind who loves strangers and all that.
Hector, my GSD, I think would protect me, though I'm not sure. We were running one night and some kids (like high school kids) jumped out trying to scare me. Hector lunged and tried to bite the closest one, and would have succeeded if the kid was a tiny bit slower running off or if my leash was a little longer. Since he reacted aggressively to that aggression, I suspect he'd be likely to protect me--but that could be wishful thinking and he might have just been startled! As a side note, it was a very dark area and I'm pretty sure they were high (I smelled marijuana in the air), so I don't think they realized I had a dog with me. I even heard one of them say, "Where the &@#% did that dog come from?!" as they ran off. LOL
The other two are reactive and are my "dangerous" dogs, as in the most likely to bite someone--but they're also the ones I think are not likely to actually protect me, because their bitey-ness comes from fear and they'd prefer to run away rather than bite--they only are aggressive if they don't have retreat as an option. So I might be safe if I have them leashed to me, but otherwise I wouldn't count on it.
Anyway, the protective benefit of dogs is as a deterrent IMO. Most criminals are looking for an easy, relatively low-risk victim, and the mere presence of a GSD is going to scare them off. I really have no fear when I'm out with my dogs, or even leaving them home alone. Heck, once before I got my GSD and I just had 3 blue heelers (one of whom was a foster and was only about 3 months old), there was a crime spree in my usually very safe neighborhood. Both my neighbors' houses were broken into and several other houses on the block were as well. The ones that weren't all had indoor dogs. Didn't matter if it was a couple of yappy Chihuahuas, a pit bull, or an elderly and half-blind Labrador (those are the 3 houses whose dogs I knew LOL), they were left alone. I thought it was a very useful illustration.