A helpful tactic, no doubt, but sometimes it´s actually somewhat of an exploit. For example, one time I was besieging the macedon capital and was attacked about six times from armies on the outside, often as not one-unit stacks. Every time the garrison sallied out, lost more men and came back out again the next. After a while it got really tedios. Without doubt, the city eventually fell to me. But instead of one big decisive battle the AI had surrounded my sieging army with six armies and each of them attacked, one after another and every time the other armies, or what was left of them, came as reinforcements.