High kicks have an arguable use in self defence although they surely display excellent athletic performance and look great. In kickboxing, particularly in full contact training, many people have adopted techniques that limit kicks level to the waist for power a speed performance: truth is, when a powerful round or hook kick connect to the head it is likely to lead to a KO.
Bill “superfoot” Wallace retired in the early eighties as undefeated world champion in the middle weight of full contact kickboxing. His combat strategy was always based on fantastic kicking techniques that often caught by surprise his opponents and knock them KO. Wallace was not just good and superfast in kicking but he could shoot double of triple kicks with a single leg, using these techniques in the same way most boxers faint punching techniques.
In this video he shows one of his legendary training exercises to help improving the central split, so useful for side, round and hook kicks: