I agree that a reasonably determined thief will have his way with most unattended panniers. Still even a very basic locking system is better than none. People attacking luggage with bolt cutters or razors tend to stand out in a crowd discouraging all but hard core and pro thieves. I've made the mistake of locking and 'hiding' my bike out of public view thus providing the thief with a perfect cover to do his/her dirty deed. If you are going to leave your bike unattended, best choose a spot frequented by honest bystanders, police even! In Asia at least, cafe owners, guest house operators, people in the service/tourist industry are only too happy to keep an eye on your gear in return for your custom; a tiny tip works well too. I've wheeled my bike into bedrooms, restaurants, tents you name it, and if anyone objects, it's up to them to offer a secure alternative - or take your custom elsewhere. Care and common sense will usually do the trick.