I know what kicks you mean that tend to connect with the shin, but it limits the force of the blow because it's spread out over a larger surface. The ball, kinfe edge, or heel of the foot is the optimal striking surface for kicks because of the high concentration of force in a small area.
the shin is a VERY POWERFUL tool to utilize in kicking. the foot is a bunch of tiny bones, while the shin is one big, solid piece of bone you can strike with. and especially if it has been conditioned, like muay thai practitioners tend to do, it can be ESPECIALLY destructive. i'm sorry but the concentrating the force in a small area "theory" is flawed in this case, because of how fragile the foot is compared to the shin.
the ball of the foot u'd use for front kicks, knife edge or heel for side kicks, spinning side kicks, flying sidekicks, etc... heel u'd also use for heel kicks, spinning heel kicks, etc...
and the shins are best for round kicks.