They're very expensive auxillary units, though most people want to use them as primary attacking units. They're good at protecting the flanks since the scythes destroy cav trying to attack. Even if they lose, they will have left the cav weakened for another unit to destoy. A great setup would be chariots with spears protecting their rear. If the chariot decides to attack a unit ahead of it and feints, the attacker will be met with the spears and then hit from behind by the chariot.

Chariots would be best at the beginning to weaken the morale and disorder the ranks and delay the enemy until the army is in position. Use them in comination with missle cavalry/skirmishers &melee cavalry watching over all of them.

On the offensive, I would hold off on them until the enemy is sufficiently broken and exhausted and then use them to create a mass rout.