Spies are useless? In MTW a spy could tell you where and when the enemy was going to attack a few years in advance! They could also cause discent in enemy nations (amongst the population and armies) and allow you to spark off a civil war quickly (usefull for removing weak royal lines). The only downside of them is they get caught too easily by border forts :(

Im not that big a fan of the Assasin, a Grand Inquisitor does the same job but in a much cleaner way