Their plan does actually. If you're not a compatible host or have an emotional trigger, the beast can take over.
Naruto was a compatible host and we saw how hard it was for him to control himself when he got emotional.
There are so many things that could go wrong with plan.
1. I'm pretty sure it was stated that most people are incompatable with Bijuu and usually end up dying. So were they just planning on abducting random ninja until they got one that fit? Why not just find a loyal Mist ninja who'd be willing to die for the cause and then have him sneak into Konoha?
2. What if Kakashi hadn't interfered? How were they planning on sending Rin back to the village? Why would she voluntarily go back when she knew she had an unstable chakra monster inside of her? Wouldn't she just refuse to go back or kill herself? (hey, waddya know? That actually happened!)
3. What if the Bijuu actually really did take to Rin and she was able to control it? Then all they would have done was just given an enemy soldier an insane chakra boost and let the chakra monster fall into the hands of the enemy.
Edit: You know.. I just thought of something though… Why did Rin kill herself?
Why didn't she just go back to the mist village and explode since she apparently had free will?
I know I would have.
See, even you acknowledge a problem with the plan.
But to answer your question, Rin, good-natured girl that she is (I guess), probably wouldn't want to kill anyone. Certainly not a whole village.
You know…why is Sanbi alive?
Don't these things die if the host dies?
Actually, that's the one part that does make sense. Bijuu go away for a while after the host dies, but eventually are revived.