No debate about keeping your hands up. There's a difference, though, between keeping your hands up when punching as compared to when kicking.
In punching, it's just good training to make it habit, but when throwing kicks it's also about resisting what the body wants to do.
Proprioception makes the arms counterbalance the kick to keep the body from falling on a sudden shift of balance - one leg suddenly leaving the ground and your body now being on one leg. It's physiology, just like when you walk on an icy patch and slip - your arms snap to a position that will try to catch your balance from a quick and sudden shift of your body line. You never think of where your arms should go on that icy step, they just go there really quick. Even though a kick is a trained movement, our arms want to go where they're programmed to physiologically. The faster you kick, the faster the arms want to move to counter balance. Look on youtube or tapes/whatever of the fights of the best martial guys around. Watch their arms when they're really kicking, there's always that counterbalance of at least one arm, sometimes both. It's especially evident in slow mo. And the higher the kick, the more you see it. We all do it. And sometimes we get smacked by a counter. It's a constant battle to keep our hands up.