actually doesnt work that way, releasing a landless courtier wont give you a bonus with your dukes or whatever. the person has to be a count (alltough i might be wrong there) or higher and he has to be one of your vassals, for example as england if you capture and release the french king, it will give you bonus what so ever (except an instant 100% warscore if you are at war with france) and ofcourse a big ransom if he can afford it