(1) First, you're wrong about "R" for violence. Many, many PG-13 movies, and even quite a bit of PG movies, have much more graphic violence. But add in just a little bit of sex (and more than the briefest bit of nudity, or showing genitals at all) and you're in R-town. One of the long-running complaints of the MPAA is how they underplay violence and overplay sex.
(2) There is no "x" rating anymore. "XXX" was made up by the porn industry, and to get away from that idea, the MPAA abandoned "X" for adults only and established the NC-17 rating. But I digress.
(3) To your question, YES, smoking should be taken into account when determinig the rating of a movie. Check out the Time Magazine article here: http://www.time.com/time/magazine/article/0,9171,1609773,00.html
"Two recent studies, published in Lancet and Pediatrics, have found that among children as young as 10, those exposed to the most screen smoking are up to 2.7 times as likely as others to pick up the habit. Worse, it's the ones from nonsmoking homes who are hit the hardest, perhaps because they are spared the dirty ashtrays and musty drapes that make real-world smoking a lot less appealing than the sanitized cinematic version."
Smoking in the movies is largely gratuitious, often used as a prop for bad actors who don't know what else to do with their hands. You may think "that's a cool shot" when you see it, but you don't miss it if it's not there.
Finally, remember, that for a long time tobacco companies PAID to have their products placed in movies. And although product placement is common in flicks, when the product is something highly addictive and unsafe, and usually glamorized, we have to think about who is viewing those messages and whether they'll be able to discern truth from fiction, glamor from reality.
Certainly, you have to take it into context-- A period piece from the 30s or 40s is probably going to have some smoking in it (although it doesn't have to be full of smoke).
(ANd yes, we can assume how ratings will dictate the film's profitability. Many parents won't let young teens go see R rated movies with them, and generally teens can't get into R rated movies themseleves. To the extent that the movie theater is the "hang around" place for people of that age, that undercuts the box office.)
So, in short, yes it is an adult activity that, along with swearing, sexing it up, and shooting people, should be taken into account. Now whether the MPAA does a good job of this or not (as discussed in "This Film is Not Yet Rated," [by the way, the movie isn't rated NC-17 is it? It's just not rated... which is why it won't be carried in most theaters]) is a different question.