Not only is heel planting unsustainable for long distances, it is also extremely uncomfortable when running any distance. If you're planting poorly in forgiving, modern running shoes, you can get away with it.  Switch to Five Fingers however, and you're immediately aware of a bad foot strike position.  For me, this kind of feedback was exactly what I needed to improve my stride.<p>For the mountain bikers out there - think of a full suspension bike versus a hardtail.  The full suspension is undeniably a smoother ride, but it's universally agreed that a hardtail forces you improve your technical riding, since you don't have all that squish to eat up your mistakes.