All righty, let me see if I can add some stuff here.
The lwo gaurd (what some think of as a lack of gaurd) is a result of Olympic-style sparring rule sets. Punches, though they should score...rarely do. All the scoring techniques will most likeyl be kicks with 80-90 percent of them being roundhouse kicks. Keeping the arms lower makes more sense as you are more likely to intercept an incoming kick with the arms lower. With a more standard gaurd, it is easier to slip a kick up under the elbows and hit the hogu.
"What about head shots?" I hear you ask. Well, those are less likely to occur in most matches and thus less likely to score. It takes longer for a foot to go from the floor ot the head than it does to get to the torso. If you do have to block a head shot, just raise your hand straigh up in the air (like Horshack from "Welcome Back Kotter") on the side the kcik is coming in from. Nothing fancy. Again, you are better off gaurding against high percentage techniques most of the time. What I always tell the students on comp team is "low gaurd does not mean no gaurd." Frankly, under a different ruleset or a self defense situation, I adjust my gaurd accordingly.
As to not blocking...not entirely true, but here's how we present that to our kids. Best is to use speed and footwork to goad your opponenet into attacking a perceived oipening...take that away and counter-attack. Next best is to you use speed and foot work to make your opponenet miss (and hopefully counter-attack). Third best is to block and counter.
Blocking gets you tired quicker. Bloking puts closer in range to get hit (you are close to your opponenet, in fron t of them in the red zone). If you get stuck in a siuation where you have to bloack a flurry of kicks...chances are one will eventually get in. If you can dodge, evade...you have more control over where you are and typically can put yourself in a better position relative to your opponent to score. It is generally better to get in, score one or two quick points then get out than it is to stay in trading blows.
Under a different reulset the tactics and strategies are different...these concepts tend to work better under Olympic-style sparring rules.
Peace,
Erik