Six of last year's ten most frequently stolen car models are Acura Integras. My 1990 model doesn't make the list, but probably only because there are fewer of them on the road than the more recent ones. At least it's a stick, so the thieves who only know how to drive automatic will be stuck... Time to start locking the door when I leave it parked around town?
Better to leave hte doors unlocked, I think. If someone wants to steal your car, they'll just break the window. Its not look people go looking for an unlocked car... that'd be too time-consuming I think.
And which would you rather have, a stolen car returned, or a stolen car returned with a smashed window?
A locked door keeps an honest person honest. Certainly it won't stop a thief, but at least if the thief needs to break the glass, it should be fairly obvious to any casual observer that something is amiss.
There's also the related issue that if you don't lock your doors, and your car gets stolen, then you haven't done everything in your power to prevent the theft of your car, and your insurance company can be unpleasant about any possible payouts (regardless of its return).