|I don't want to glamorise this filthy habit, but HHHNNNNNNNNNGGGGGHHHHHH|
Many players and DMs (myself included) tend to use smoking as a quick characterisation - someone who smokes is generally a rebel, too cool for school, and (depending on the setting) generally a bad guy. It can make a one-dimensional character have the illusion of depth (but mostly makes them smell bad). It's also an addiction that allows someone to remain fully-functioning (unlike drugs, or in some cases alcohol), and gives people an excuse to be an arsehole when they haven't had their fix (though I've met a fair few players who didn't need an excuse)...
In games with "Flaws", Addiction (Smoking) is a cheap and effective one - you don't buy a single dose, you buy a 20-deck! And they work it as "one dose a day" for most addictions, so you're fairly guaranteed (so long as you are somewhere near civilisation) to have a steady supply of your chosen vice.
|This is how I Flaw: Crippling Addiction (Tobacco)|
I also had plans to make them the motivation in a short one-off one-on-one Risus game to introduce a friend to gaming ("You've smoked your last cigarette. You must adventure out to the shops, to buy more. It's dangerous to go alone - take this." *offers kitchen knife*)
And, as a DM and player, I've had several in-character smoke breaks, including plotting with the sole other Elf in the party as to how we would be the ones to retrieve the McGuffin for our Queen (and then, out-of-character, how we would never let the other one live), and using it as a chance to let the players work out the terms of a deal (with one player approaching me in-character and making a better offer for her services, and the rest of the party as slaves). It can be a lot of fun, and add some vermilisitude, to smoke while gaming - just make sure you're in a smoke-friendly house (most of my gaming friends are not).