That is why it surprised me to learn that quite a few cyclists I know apparently do this several times every day and will not leave their bike outdoors for anything beyond a quick errand. Their concern is not safety, but durability. One acquaintance tells me that leaving her bicycle outside - not overnight, but during a normal workday - leads to rust and mechanical issues, particularly if it happens to rain. Another cyclist complained that after half a year of leaving her new Dutch-style bicycle outdoors 9-5 on a daily basis, the bike is now in such bad shape that the shop she took it to for a cleanup and tune-up told her not to bother and just buy a new one. It sounds absurd, but I have more anecdotes in the same vein, all involving bikes purchased in the past 2-3 years: It seems that many of the new wave "city bikes" - unlike the European originals that inspired them - were not actually designed to withstand the outdoors.
Of the current-production bikes I've owned or had on loan, I have kept a Pashley, a Bella Ciao, a Pilen, an Urbana, and a Paper Bicycle outdoors for extensive periods of time and have observed no damage as a result of this practice. Same with the vintage bikes I've owned - my Gazelle and the Steyr I rode in Austria both stayed outside overnight and were none the worse for wear. So what did these manufacturers do differently, and is it possible to do the same to other bikes aftermarket? I suppose a frame can be sprayed with some rust-proof solvent, but what about the components?
Finally, I am curious to know what you feel is realistic to expect from a new transportation bike, as far as its outdoor durability. Should it be rated for being left outside for a couple of hours at a time? A standard 9-5 workday? Overnight storage? Your own experiences with specific bikes are welcome.