And then there is the aspect that throwing continual kicks is draining. Just think, when fighters (MMA, boxers, etc.) are punching and as they get tired or fatigued their hands start to drop. The majority can realize this themselves or hear coaching from their corner to bring their hands back up. Now perhaps this was the case in TKD that eventually it evolved to what we see now that many do not raise their hands at all. In my case when we TKD spar we are encouraged to keep our hands up, and we are WTF, but as you get tired as kicking with your legs is far more tiring then punching, that I start to drop my hands and then I hear from the outside to put them back up.
In fact with many of our kicking drill classes we are told to keep our hands up even when going to the back of the line. If one person drops their hands then the whole group has to doing monkey jumps or clapping push-ups. Of course for many school that are just sports orientated that may not be an issue but for a school such as ours that is more geared towards self defense, hands up is a must and is practiced frequently. Punching across the gym ensuring that your jab hand comes back quickly to protect the jaw while the power hand is up blocking the side of your head. These and many other drills just hammers that home.
And don't kid yourself about it just being TKD or just WTF. I have seen ITF people like this and have seen others in Karate too. It depends on the person, where they train, etc.
"IF I COME ... I'M BRINGING THE PAIN WITH ME"