Probably also depends on what they are supposed to do. Having a bunch of low-level spearmen in guard mode seems to let them survive longer, but maybe won't be able to deal as much damage as they otherwise could, but since their purpose is to hold the enemy in position while other units move to attack the enemy in the flanks and/or from behind, it is a fair exchange. However guard mode is not such a good idea for flanking units, since you want them to kill the enemy as quickly as they can, before they can turn around.