How Effective were Spears Against Cavalry?

[Macedon]

Not really. They did not commence any charge - it was a feigned charge, as you clearly described.

Testing enemy morale and distracting enemy attention from own horse archers cost them nothing.

That's why there was no reason not to carry out a feigned charge to test the enemy.

?? A "feign" charge has to commence or else it is no charge... A charge is a charge even if it does not end in hand to hand combat. They did so, hoping that the Romans would not sustain their attack. The Romans did, the Parthians stopped before the "clash". The Parthians retreated. Had the Romans not withstood the cataphract approach and lose their order, their cohesion and present gaps and disorder, then the Parthians would not have stopped the charge but they would have attacked... The reason to do things on the battlefield is because they may work. If you see that they do not work, you recall, regroup, try something else. This is why they conducted the charge, not having a reason not to do something is not a reason to do it. In a battle, you need reasons to do stuff. There was also no reason for the Parthians to not dance the Harlem shake but no source says they did...