[aNgRyDuCk]

on a serious note, and very simply, if in fact you offer what a player needs, as an alliance, he'll stay. If you do not, either because of misrepresentation, misgivings, personality clashes, ambition, or lack thereof, either with respect to the player or the alliance, then the player will likely go elsewhere when presented with a better, or atleast, a more likeable situation for them.

If another alliance presents an opportunity to a player, and he isn't getting what he/she expected or needs from the current alliance then you certainly cannot fault that player for moving. In addition, you cannot slam alliance recruiters because they are on the look out for players who will improve their aliance, as that is in fact what their job is.