I recently had the barrel of a bike lock sieze up on me badly. Unfortunately it was when I'd come back from the supermarket, carrying two panniers full of groceries to be ridden home... including an unfrozen chicken. In the middle of a very sunny day, and too far from home to walk home with so much groceries.
So I took a cab home and went to the hardware store to get some supplies. Went back to the bike, which was still locked, in the middle of the shopping centre. Graphite in the lock's barrel- wouldn't unlock. WD-40 in the lock's barrel, knowing full well it may destroy the lock- still wouldn't unlock. So then I got out the hacksaw. In the middle of the shopping centre. Which took no more than five minutes. Then rode it home.
The eye opener for me was that although I got a lot of very odd looks throughout the whole affair, not once did anyone ask waht I was doing, or call security on me. I could have stolen someone else's bike in broad daylight, in the middle of a shopping centre, and nobody would have pulled me up on it.
I agree- anything can be stolen. All locking up does is deter.