I actually read this interesting Inception/Animorphs crossover fic set post-54 in which there was a system through which voluntary hosts would "contract out" Yeerks for their own advantage, which if you think about it makes sense. A Yeerk can control functions that are involuntary in humans, like breathing and heartbeat. A cooperative Yeerk could double your thought-processing power, because both the host's and the Yeerk's brains could be put to the same problem at the same time. So if you're, say, a point man for dream heists, or a soldier or something, you might want to hire a Yeerk, as long as the Yeerk is held accountable for its compliance with the contract every time it goes back for feeding.

EDIT: Here it is (http://cthonical.livejournal.com/97227.html?thread=1362379#t1362379).