I for one believe in the idea of campaign loyalty -- if I apply for a campaign, it means I plan on being part of it for a long time, possibly until it ends. If the manager sees fit to accept me into it, I feel they deserve for me to stick around for at least a while. Sometimes managers of campaigns I'm in have offered other campaigns they have available to me, in which case they are the ones suggesting that I move; in that case its fine.
But I know its just additional work for them if you move around a lot, and then old posts no longer carry the signature, which means no new traffic can be directed to the sponsor's webpage.