Watching each other's back is quite a misconception. If there was a fully fledged battle where the width of the army was matched by the other one, the need for watching the back was pretty useless. Sometimes spearmen were often used in the front line, in order to dissolve the shock of the frontal charge, and then let the knights a row behind to do the killing. Many of the knights were fairly well equipped as they were heavy infantry which was required to do most of the killing.