Why take the chance when it is so easy to protect yourself?
Better to spend 5 minutes setting up a wpa-enabled password then 1 week trying to prove to the police that you had nothing to do with illegal stuff transiting your router.
I hear what you are saying, but really, save yourself a potential headache.
For the same reason I don’t lock my car keys in a safe at night to keep people from taking my car. The chances of me forgetting the safe combination are greater than my chances of someone taking my keys.