I think this would add a lot to the game environment, especially in Taverns. I'd like to be able, when I am afk, or while listening to a bard's tale, to have my character smoke his pipe.. Not related to this thread but the idea occurred to me as well that bards should be able to play their instrument while afk, or just set it to auto-play.
Smokable substances would be a fun RP feature. Real-world smokables such as tobacco, cannabis, and crack cocaine should be left out, and instead fictional herbs should probably be used. These herbs could be grown in agriculture, or foraged from wild plants, so half the implementation is already taken care off.
It definitely feels weird that everyone is guzzling down tankard after tankard of ale but no Bobbit's leaf in sight!