Many martial art styles, especially Tae Kwon Do, kick more often than others for a reason. The leg is far longer than the arm, giving it a greater reach which in turn gives the person kicking a greater advantage over their punching counterpart. The leg also contains one of the biggest and strongest muscles in the body, giving a kick three to four times the power and devastating impact that a punch can carry. And kicks can be centered in on areas that are difficult if not impossible to defend--knees, groin, femur, tibia.
The Korean style of Tae Kwon Do can be a little more damaging because they roll the ankle forward, using the heel to deliver more power to their blows then the Japanese or Okinawan styles.
Although kicks can mean the difference between winning and losing in an attack, improperly executed kicks can put the odds in an opponents favor. Martial artists are taught to maintain balance by keeping the center of gravity within the boundaries of their support system, and to keep their eyes on the attacker to make sure the kicks are striking in the intended place. Martial artists--especially Tae Kwon Do practitioners--use all their strength, weight and momentum to kick someone, then snap the foot back to keep it from being grabbed. Martial artists also remember to keep their hands up for defense and move quickly without telegraphing what their intent is.
Though the advantages are great, there are also some disadvantages to kicking. The legs can be a bit slower than the arms because they do have farther to travel. Also, because people tend to use their hands more than their feet for touching and grabbing things, it’s easier to touch an intended point with a finger than a toe. It is also much easier to become unbalanced on one foot while kicking than two feet while punching.
In the end, before executing a kick in self-defense, one should make sure the advantages outweigh the disadvantages.