Piercing spear can't compete with other classes that have the same distance ranged attack or classes that can close the same distance with a super armor. The flow should always be used because you don't know if it will knock them down or not. But if it doesn't then it lets you chase after them to close the gap and knock them down. The problem with the flow is that it has no forward guard. So if you hit someone with the piercing spear and they have SA up. They will knock you down as you charge up to them because you have no defense.