Would you leave a safe closed but unlocked? It's no different than leaving a wallet unencrypted.
When the safe is located in your locked house and surrounded by a security fence you might leave it open.
Not quite the same as if the safe has been left on the street is it?
Your house is your computer so obviously your computer (or house in this example) seems to have a lot of open windows, else noone gets in. The safe (encrypted wallet) is last line of defense.