My U-Lock finally got rusty, but that was after many Burns and forgetting to clean it (in whatever way it might actually
be cleaned). I could still technically get it to lock (most of the time

) but finally simply chucked it. I bought a number-combo cable lock for this year so I don't have to worry about keys.
In a pinch, I've even used a heavy length of chain and a padlock. I've always locked my bike, if only to itself. Never a mishap; knock on wood . . .
A really determined bike thief can probably get through almost anything if they have come to steal, but most bike thieves out there are
not determined, they're just drunk. (And tired, and want a ride across the playa to their tent.) Or, they're confused about the identity of their bike, because they've only owned it for 3 days and it's dark.
Hedge your bets by bringing a cheapo bike (bigger tires are better) and locking it even when only away for 30 seconds. Drag it away from the street and behind something when not in use: taking a bike that's two feet away from the street is one thing; creeping through someone's stuff at length takes someone more unusual and rare.