I find mercs to be a usefull asset especialy when you need to get some men on the line fast. I find some times I will hire mercs on the way to a city to bolster my ranks. Some times I send a general out and the armie that he gets is waht ever I can aford to buy on the way to where he is going. It is not exactly the best tactic, I know, but it dose get the job done in a pinch.