I have been told that some okinawan style only taught the front kick. This fall my Okinawan instructor taught us several different types of front kick based on the "old" way of doing things.
If I remember correctly they were all done with the big toe-proper conditioning required. He can literally get up on his toes and walk across the room.
One kick involved raking across the leg with the intend of cutting the skin with your toe nail. The nail being dirty and these techniques being before the development of anti-biotics the chance of infection was good.
Another involved curling the toe over and behind the crease of the junction between the leg and lower torso hitting a nerve that hurts and makes you want to sit down.
It would be interesting to know the history of the roundhouse kick. Did it start as a slapping kick (with the instep) then evolve into a toe or ball of the foot kick, was it the other way around or did it evolve differently depending on where it was "invented".