What if we evolved as parasitic slugs? Wouldn't you want to be able to see, hear and feel the beauty of our world?
Please don't stereotype Yeerks, just because you see one bad Yeerk, doens't mean every Yeerk is. It's pretty obvious actually. What about terrorists, rapists, kidnappers, and such? What if "Andalites" came to our world, then saw terrorists, then label of all humans as evil, would that be right? That is what you are actually doing right now.
We treat livestock and beats of burden with more respect and care than Yeerks do with hosts.
Have you dissected a frog? Do you know how many frogs we dissect every year?
Have you seen the human-cow? The hybrid between the cow and human? Isn't that worse??
At least the host body lives to its full potential (if the Yeerk is friendly).
Or the host body actually lives healthy (any Yeerk, evil or not).