I have various removable packs/bags adorning my different bikes. The most valuable item is the smallest, my trunk bag with tubes, folding tires, pump, patch kit, front & rear lights, and tools. I move it from bike to bike, contents included, as I change mounts. If I'm locking up in the city, I take the most attractive & critical items with me, i.e. the lights. I've never had anything stolen, much less a whole bag/pack/pannier. But then I've never locked up (or needed to) in what would be considered a "bad area".