Well it would make sense that they ply their trade in undeveloped lands. Why would you hire them when you get them for half price by training them (In your well developed provinces anway). I think they just move around I guess if they don't get hired they move off like in real life. I have definately seen the smae unit travelling the med a few times in games.

I try not rely on them too heavily anyways. But if there is that choice unit to fill a gap in my armies I always snap it up quick. Normally it'll be the quality Sword units or polearms. ie Druz. . . (cough) Feudal Foot Knights (cough). . . inha cavalry, Billmen or some Vikings