I bought some really cheap panniers today - around $20 US. Put them on the bike, and discovered that the carry handles touched the rack. So, I dug out some really tiny padlocks I bought (on of those "No idea what I want this for, but they're so COOL" purchases abour 2 years ago), and was able to lock the panniers to the bike. It wouldn't deter someone that really wanted them, but who's going to bother breaking a lock or cutting a handle to get $20 panniers off my rusty beater? No one.