The vast majority of the people I know my age smoke on ocassion, no more. I know tons of people who smoke at parties only, or during finals weeks, or one person who somes once a month.
Now do note, I said in my post above the cigarettes are much easier to become addicted to than cigars, pipes, snus, etc. The reason being that you can do them quickly and often. A cigar takes time. A pipe takes work and isn't the sort of thing you rush out during class break to smoke. So cigarettes can more easily become a habit, just on a practical level.
Then there is this, that the vast majority of cigarettes have additional chemicals that are harmful to you and make them more addictive, but not all cigarettes have this. There are people who have an easier time quitinng by first switching to natural cigarettes for a while, as their bodies are able to lose dependence on these other chemicals first, because one gives up the nicotine, or frankly (where most addiction comes from) the ritual. I have no trouble not smoking at home with my parents and most places, but with certain people and in certain places I smoke more frequently and slip right back into a habit of it. The addiction there is not the nicotine, it is psychological. I have found that I can just as well puff a cigarette in such situations. It is the psychological habit, especially in social situations, that is compelling there.
The weird thing is, statistically, is that there is a higher proportion among "smokers" today (referring to cigarette smokers, not cigar and pipe) that smoke more than there were in the 1950's and 1960's. My guess is that the truly light smokers who were a higher percentage of smokers back then are precisely those people now who can take and leave it (the many, in some places majority of "non smokers," who do, actually, smoke here and there but not regularly). Back then it simply happened all the time in social situations. So the truly light smoker is not even counted anymore as a smoker, and of those who continue despite stigmas you are more likely to find those who get addicted more easily and hence smoke more.
Addiction is, in itself, a spiritual imperfection, not a sin. But to wilfully allow oneself to get addicted when it can be reasonably foreseen and is unnecessary (unlike my medicine example above) would be sinful, as one would be choose the imperfection voluntarily.
For the record to, I know plenty of regular smokers who do not smoke when to visit their parents back home, even when it is a whole month. They don't have any cravings or anything of the sort. But back in Berkeley, or wherever they are used to smoking, they do crave it. A lot of this is psychological and varies widly from person to person.
I would still maintain though, that since taking good care of our bodies is a duty, that to intentionally and habitually do something that has a significant harmful effect on our body ranges from imprudent to serious sin, depending on the circumstances. Most people believe this to be true about smoking
Now again I repeat. To not smoke because one has an addictive personality, or because they do not like it, or have no desire to is not a sin. To smoke when it is adversely affecting your health in a notable way, without just cause, is a sin. To not smoke because you think that smoking is a sin is an error. It is not thinking with the Church. To persist in that error is a sin. Yes I said it.
Catholic moral theology says it is not a sin. Someone who says it is is not thinking with Catholic moral theology. That should be realized.