I live in Bucharest, which pretty much is as bike unfriendly as you can get. There are some bike lanes, but all of them are on the sidewalks where you might see a car parked or you might have to avoid a tree or a bus station, so they are pretty much unusable in my opinion. On top of that, there is a large number of aggressive or simply ignorant bike riders, so you can occasionally get some strong words simply because you're riding your road bike on, you know, the road.
That being said, I never wear a bike in the city. There are a couple of reasons for that:
First, it hinders my sight. I'd rather be able to catch a glimpse of a car trying to overtake me than have my head protected when I hit that car. Second, most road bike helmets offer some sort of protection only when you hit the road with the top front of your head. They offer little protection when you fall sideways, which is probably the most common type of fall. Third, I'm not so sure that a helmet might do anything for me: I ride a bike, ski and even skate a bit and I've fell down a lot, but I've never hit my head once because my instinct was to protect my head with my hands.
Last, it's a matter of personal taste: I sweat a lot and I'd feel awkward if I had to wash my hair every time I ride my bike somewhere.
That being said, I always wear a helmet when I plan a medium or long ride. I usually ride faster than in the city and a helmet might make the difference. Also, I always wear a helmet if I'm riding my mountain bike in the forest because there are branches that seem high enough, but guess what, they aren't. I also wear a Road ID, just in case.
As a bottom line, I strongly believe that bike helmets don't offer safety. A little, yes, but hitting your head hard where it's protected by your helmet is just one of the injuries, especially when you ride next to the curb. I think it's a lot better to watch out and try to prevent any accidents.