I understand if you were just thinking in the basic sense of a parasite and host, but since both species are sentient and complex, it's not as simple. I don't think Cassie said anything to Aftran about the sharing, and how many of the people that come there are voluntary in book 19. Yeerks don't take involuntary hosts because they have to. they do it because they want to. Their government is a large part of the problem. At once point it;s stated that 60% of new hosts gotten through the sharing are voluntary. I don't know how many new members they get a week or month, though. If you just look, you'll see that if they had done things peacefully, they could have gotten what they wanted. It's just the council didn't want hosts for their people. They wanted conquest for themselves.
This is why I sympathize somewhat with the Yeerks. This is why I don't just ostracize voluntaries as scum. this is why I disagree with making the Yeerks species into nothlits, whether they like it or not. Nothlitizing the Yeerks, and Taxxons as well, was a Cassie idea. Even though some were up for it, it didn't make much sense. The taxxons who lived with the Living Hive weren't raging beasts. Yeerks can live with hosts without hurting them, as long as they respect their hosts. I know that the Taxxons came up with the idea to become nothlits, but to me it just came out of left field.