Change
It can have the effect of allowing you to change the kick. This sacrifices power though, as you are reserving some follow through for the sake of having a compound strategy or a fallback.
I throw something that is between a thai kick and the typical kyokushin gedan mawashi geri. It allows me to hit thoroughly and deeply but also allows me to change it a little if the target changes (if I am a little bit lucky). My impact shape of my leg is pretty similar to a thai kick, and I do stay on the front 1/3 or ball of my foot and never plant the heel. It is my own personal kicking style that I have melded together from various influences.
I tend to hit with the shin right above the ankle flexing point, but can use the instep to slap if things don't go right, causing pain and annoyance and this can even bring unconditioned people down.
I think that the keeping of the heel down in Kyokushin is also a flexibility thing. In kihon the idea is to develop very strong hip flexibility. Unfortunately unless a person starts in their teens when ligaments and tendons can still be shaped, it will just result in damage to the knees for most people. Keep the heel planted and learn to use all the small muscles of the hip and legs to extend and gain maximum power.
Glenn R. Manry
---Iaijutsu, don't forget the doorman.