How important was individual skill in formation fighting?

[Americus]

A soldier in combat must be competent in his use of his weapons in order to defend himself and inflict casualties on the enemy. If he is flailing around with his weapon and shield, he won't be of much use. His peers won't trust him. He also must have confidence in his ability to fight effectively and survive, and that of his follow soldiers standing in rank with him in order to have the willingness to stand and fight instead of running away. When he has done this in combat and become a veteran, he becomes even better. That experience and confidence is important in instilling the discipline to remain in rank and fight.