Thieves don't wan to be caught or injured - they'll go for a house that doesn't have a dog bark at them when they are peeking in the windows; they'll go for the house without an alarm box next to the one that does. Even a small dog works for deterrence even if not so much actual physical defence - I've known people with some very non-aggressive dogs who still have 'Beware of Dog' signs for this exact reason.
Thieves don't wan to be caught or injured - they'll go for a house that doesn't have a dog bark at them when they are peeking in the windows; they'll go for the house without an alarm box next to the one that does. Even a small dog works for deterrence even if not so much actual physical defence - I've known people with some very non-aggressive dogs who still have 'Beware of Dog' signs for this exact reason.