So you wouldn't mind losing your body and all your secrets to an alien slug as long as he wasn't a complete psychopath? As long as he's only a little bit evil you wouldn't mind? What difference does it make?
Yes your friends and family probably will still be killed/infested, but at least you won't be the one that betrays them and has to watch as your own hands help drag them kicking and screaming into the Yeerk Pool.
As for the free will to think, you have a point, but it's hardly a reason enough to live. Imagine being trapped in a nightmare that won't end; being able to think about things other than the nightmare isn't going to be much of a consolation. And what would you say to your Yeerk exactly? "So, what's on the agenda today? Going to kill my family or shove my best friend's head into the Yeerk Pool? Anyway, what's your favourite movie?" We know what a typical Yeerk is like, constantly babbling on about how powerful they are, what species they've conquered, their evil plans for the future and how all humans are the scum of the Earth. Fascinating set of conversation topics.
Go easy on the host? Tell you what, I'll shoot your best friend with a Dracon Beam instead of shoving a slug in his brain? Well, you haven't screamed at me as much today, how about to make up for the fact that I've stolen your body and killed everyone you care about, I'll buy you a pizza?
I'm sorry if any of this seems kinda harsh, but I just find it hard to believe that anyone would so willingly accept a Yeerk in their brain instead of death.