Well, its not so much the dog itself that is unpredictable, its the situations the dog might encounter and the unpredictable ways he might react to unusual or possibly provocative situations over which you have no control. It's hard to judge exactly where a dog's line is crossed and for what reasons - I knew a dog that was sweet as pie all the time, but if he saw hardhats he turned into The Beast From Hell, slavering jaws and all, and without warning. This facet of his personality went undetected for 2 years because he never saw anyone wearing a hardhat in that time.
I always hear people saying "My dog would never bite anyone, he's such a suck" but then after an attack you often hear of people saying, "We don't understand how this could have happened, our dog has always been so sweet and gentle!"
Anyway ... while some owners are more aware than others and all that, I think it's a good rule of thumb for most people. Not everyone is equally aware or equally able to judge body language and some incidents can occur in a split second.
I don't mean that everyone ought to treat their dog like a loaded gun and keep it away from people or be too cautious either; I knew one dog that was handled this way and never got properly socialized as a result.