I use smoking in characters to show a multitude of different things. The shorthand personality indicator is very useful -- it marks a character as anything from self-destructive to a free thinker. Yep, that's right, a free thinker. In a society which is rapidly moving away from celebration of individualism (agree, don't agree, I don't care) to a form of collective nanny-ism at best and fanatical biofascism at worst, a character who smokes essentially defies all that, and smoking is an excellent piece of shorthand for showing that rebellious nature.

Ah, be careful, expressing such opinions can be dangerous. 
  Heck you might even be accused of being a reader of
Jonah Goldberg's Liberal Fascism.