Quik, I am open-minded. (Of course, that doesn't change the fact that long-term smoking does cause lung cancer.)
I think I see what you're saying. It definitely is a part of culture, albeit a bad part of culture. For instance, sometimes it's nice to hang out on the front steps on a cool night without really doing much of anything. But if you were to do this, in a completely idle manner, (i.e. doing nothing but sitting on the steps like Confucius), you would be viewed as somewhat weird -- you'd be hard to comprehend. Some people may judge you as being depressed. Others may view you as creepy. So our culture found a fix to this: if you're seen outside at 10PM smoking a cigarette, people wouldn't think twice about it. They may think you have an unhealthy habit, but they wouldn't puzzle about you and say things like you're depressed, creepy, awkward, out-of-place -- it would be familiar to them. It's similar for parties, too.
Of course, that does not justify it. You can just part with culture without picking up an unhealthy habit. More specifically, I object to smoking because:
[1] It's not worth the risk of addiction. (Even though I've never smoked before, I think I can say with some certainty that you downplay the effect of nicotine addiction. After all, it is a matter of neuroscience, which is probably more reliable than a single person's judgment.)
[2] Even if you do it for a few years and then quit, the process of quitting can interfere with your life in a very destructive way.
[3] I don't like the principle of giving in to the destructive aspects of society in order to conform. I think that most people who smoke probably wouldn't call it 'conforming', out of their own self-love, but that's really what I see it as. You are risking your health in order to acquire some vain social advantages. (The goal of acquiring transient pleasure is different -- I wouldn't call it conformity -- but it is just as nonsensical, because of [1] and [2].)
Sorry that this post is long-winded. I'm just interested in the reasons why people smoke.