Theft vulnerability varies hugely by location. To make a blanket statement about locking-up--either way-- is foolish.
I fully expect that some day, something will be tampered with or stolen from my bike. Or the whole thing will be taken. I do my best about locking strategies and securing accessories, but I don't let the threat keep me from using my bike for commuting, shopping and errands.
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+1
I expect to have a bike stolen/vandalised some time or other (past experience has taught me this), I just try to have a secure locking setup so that it doesn't happen too frequently (in my experience I get a bike stolen about once every five years, but my locking practises have improved).
Having a bike stolen is a bit like losing a friend, but if you had good use of it, then you've got an excuse to replace stuff.
Since my rims are wearing out, I almost considered not locking up both wheels to encourage myself to replace them - but I don't want to support the theft econmy, so I don't do that. However, I suppose that then the person who gets the stolen goods will have rim failure (maybe it is a good idea
)
This philosophy definitely keeps me from forking out more than about $500 on a bike, because I can't really afford to replace much more than that at this time.