Divers say "you don't have to outswim the shark, you only have to outswim your buddy".
The same logic applies to locking a bicycle. If you're bike is worth more than those around you, it's the more attractive target. Likewise, if you're lock is better or harder to defeat than those around you, the thief will move to an easier target.
So it's not about preventing theft, only about preventing theft of your bike, by encouraging the thief to pick a better target.
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