Jim has a good point in that post of his...open hand techniques. Nothing has ever stopped me as quickly as a smack to the nose or an open-handed backfist to the nose/mouth area. We all know the average person doesn't like getting hit in the face and instinctively pulls away. I do it to my pets (dogs in this case, cats in my ex-gfs) all the time (no! Not hitting them! :P) but try blowing in their faces. They have a natural instinct to pull away and rub their faces checking for "damage" before coming back for more. Its an unsettling sensation.
Another point. Personally, if I thought I were in trouble, the LAST thing I'd do is punch to the head...not so much because I'm worried about their well-being, but because no matter how trained I am, I KNOW a small mistake is going to mean a shattered hand, wrist or worse. That's one BIG bone on our shoulders...and damn hard, even on the jaw! Unless you're really sure about that hit, I wouldn't risk it. There are far more vulnerable parts of the body I'd go for such as the stomach/ribs, thighs, shins, groin, feet, knees.
In short, headhunting is good in sparring/ring fighting because you're LOOKING for the knockout. But in REAL fighting, it's probably not a great option.