Ah, well -- it's simple. The neck is a tool our body uses for surveying our surroundings. It allows us to (in a hunter/gatherer sense) run whilst looking around/behind us. Without the neck, we would be very vulnerable to all sorts of things. Heh, kind of like how Batman was with his original armor (in the Dark Knight) which really limited his mobility. It's essential that if you're going to be the top predator -- you have to be able to look around very quickly.
Could you imagine how hilarious/awful it would be if we all ran around with stationary necks? If we wanted the 360 on whatever environment we're in -- we would have to stop and turn our bodies 360 degrees. That would waste both calories, time and patients.
Our bodies do however protect the most vital structure of all -- the brain. I've seen all sorts of head trauma/things that shouldn't be inside the skull. You'd be surprised how tough the human skull is.
Besides, try naming another animal that doesn't have a armored neck (besides the armadillo). It's just essential if you want to survive in the wild.
tl;dr Thick "armor"/bones is fine for both your skull or ribs, because both do not move much. We need a flexible neck in order to do things like spot predators before they can get us. Or spot prey animals so that we can bash their heads in with rocks (in the most primal sense).
And there you have it.
:EDIT: I just wanted to add -- humans (and all animals with necks) instinctively protect the neck. This is why your chin naturally drops a bit as you put your dukes up before a fight. There are loads of other examples -- but that will do just fine. I hope this all answered your question in a timely manner.