Katana makes some very good points. Many of the people answering seem to be not used to kicking with the ball of the foot. In the 41 years that I've trained every style I studied would do front kicks and round house kicks, with the ball of the foot. The only time we kicked with the instep was when we were play sparring. That was only to prevent injury. It is easy to kick with the instep but it spreads the force of the kick over a larger blunt part of the foot. That is safer for the person kicking and the person being kicked. Sure it can hurt, but the same kick done with the ball of the foot is more damaging to the person being kicked. Kicking with the ball of the foot means that you must train yourself to strike a target while making sure that what you are striking is at a good angle to you. If you kick wrong you will bend your toes back injuring them.
Note: In Okinawa it is also very common to train the students to kick using the tips of the toes. Most people assume that to have an effective kick you must kick as hard as you can. With toe kicks you only need to have a little power and use the big toe and the one next to it to kick nerve centers on the attackers legs, and thighs. I regularly practice toe kicks. Because of the regular practice I can easily kick holes in cardboard boxes leaving only a small hole the size of my big toe and the one next to it. When teaching my students if one gets to cocky, I will kick the inside of their thigh using a light toe kick. It usually will cause their leg to go out causing them to fall down. what is amazing to me is that not many styles use the ball for the foot when the practice. seems that commercialism and laziness has replaced the kick with the more common instep version. It is really a shame that we all train to be the best and strike to do injury if needed. Then we spend all out time kicking with a blunted, less effective instep kick. Then when a real self-defense situation comes up we kick the way we have been training. It is no wonder that some people that are well trained fail to stop the attacker due to improper training methods.
P.S. Before everyone gets all upset, let me say this..... I'm not saying that a kick using the instep can't be powerful. I'm only saying that the same power using the ball of the foot is more damaging. It does take more time to learn to do it well and not injure you toes. The time is well spent and becomes second nature after a while.
EDIT:
Sensei Miller....... I'm surprised at you. first you make a comment about it being stupid kicking with the toes. Obviously you have never been on the receiving end of a properly done toe kick. Some of the Old Okinawan masters were known to walk on the tip of their toes like ballerinas do.
Second I find it interesting that you practice a Korean art yet use a Japanese ID on Y/A..... Why is that? Not putting you down, just wondering!!!