Mercenaries are one of my disappointments with this game. They should have far more free-will, and should have different mechanics than levies or retinues. Instead of just hiring them, paying them a wage, and having them do whatever you want, their usage should be far more limited. For example, you should only be able to hire them for limited periods of time, to fight in a limited geographical area. They should then engage enemy troops within that area of their own volition, and not be controlled by the player at all. Once the limited period is over they should be available for re-hire (to anybody, including your enemy), and if not re-hired should fight anybody they want or siege any holdings they want. They should always be on the map, whether hired or not. Also, their numbers should deplete and not replenish, but when destroyed a new band should generate in or near their home territory. They should only be available to hire if they are within your territory or that of your enemy, and should travel towards any areas of conflict looking for work.
It'll never happen though.