Have you seen the movie Braveheart? If so you remember the first battle scene where they did the heavy calv charge and all the horsemen on the British side died off.

Well, now think if William Wallaces group of Highland Clansmen were engaging the British and that same squad of Heavy Calv hit them from behind - think of how much Chaos and raw death that would cause.

Same thing here, sure you can use Calv for attacking the front line - I often do with Chiv Kn. but they are best used to cause the enemy to snap and run from hitting from behind.