Poison can kill the pretty little kitty the nice old lady in 3C has by her pet eating the poisoned mouse before it dies. I'd feel bad if I caused that. Plus, you get a nice fat rat or pack rat, and it dies in the walls space in your bedroom, you have either eight weeks smell of death, or the equivalent of 8 weeks pay for someone to come locate it, and rip it out of the walls. With the water trap, the mouse carcass is disposable, and it doesn't kill the little kitty in 3C.
Steve