Debian, I understand your concerns. Since my dog was attacked by another dog a few weeks ago, I became very nervous taking her out, especially off the lead. I now take a stick with me should another dog do the same.
However, this was a provoked attack where the 'owners' and I use that term very loosely, thought it would be fun to set their staffie on mine.
I must have lived a very sheltered life because I have never had a random dog, off the lead, chase or indimidate me in anyway, aggressive or not. The majority of dog owners take control very seriously but like Crackle pointed out, military precision is not always possible, especially if the dog is concentrating on a squirrel/bird/scent.
IME, a good dog, a controlled dog, will not bother going anywhere near a stranger, unless the owner brings the stranger into the the social group, ie stopping for a gossip.
The root of these type of debates is always the same - there are good and bad dog owners. I wish the bad could be wiped off the face of the earth, but alas, that's not to be. I would like to see stricter laws in regard to dog ownership but I'm not sure how these could or would be policed.