That's what I thought about mine until one afternoon I answered the door and the man outside was (well, it was my impression) a bog standard scrounger at the door. Mutt was snoring softly on the sofa when I opened it. I realised very quickly that the man needed to go away, as he was shifting closer to the door as he spoke - I always stood behind the door rather than open it wide, so I wasn't obviously doing anything different to any other time I'd opened the door for a delivery, post or workman.
Suddenly the deepest, evilest growl came from what seemed to be both from my shoulder height and the deepest depths of Hell.
My derpy, daft as a brush, cat battered Staffy cross had woken up, listened, decided something was off, crossed the room silently, jumped up onto the table without knocking everything off, balanced his front paws silently on the baby gate (without falling off), not lashed his perpetually wagging tail and then waited to let the bloke know he was there and not happy in a way that made him sound like the biggest, nastiest, doorstep-conman-robber-eating beast in the world.
I saw the colour drain from the man's face and instead of being way too keen to get his toe over my threshold, he suddenly needed to leave and practically ran away.
He wasn't a guard dog. I wouldn't have a guard dog. He was a family dog, as well trained as any Staffy can be (through food and hand signals, mostly - even the baby/toddler could make him sit and roll over before she could talk from her position of authority in her highchair). He never growled or showed any hostility to any other human, child, other dog, cat, baby bluetit or anything else (except a sofa cushion once, which I suspect meant there had been an abandoned half biscuit). His life was people, running at top speed and football. Was never anything other than happy, lolloping and hungry before then and for the rest of his life.
But he meant business that afternoon.
Anyhow, you don't need a guard dog. You definitely don't need a big guard dog. A dog might be nice if you're looking for a family one, but not for protection.