Even though gluttony and drunkenness might be difficult to prove, they are, in fact, the offenses that are spoken of in the Bible as sins....along with laziness and gossipping (and we all know how many JWs just LOVE to gossip.)
Smoking, on the other hand, is never specifically mentioned, but falls into that gray area of defiling the body. What I find interesting is that the act of smoking has been around for thousands of years, and it was certainly practiced in Biblical times, although probably not tobacco specifically, since tobacco is a new world plant. The Mayans, however, DID smoke tobacco. Cannabis was most likely the plant of choice for smoking during Biblical times.
So, it would appear that the Bible writers did not consider the act of smoking to be sinful, or even defiling the body. Until the 60s, smoking tobacco was also considered to be a very healthful habit, and was even recommended by doctors at times.
Until evidence came out that cigarette smoking could cause health problems, JWs were allowed to smoke without repercussions. I find it interesting that smoking was banned only when the world at large began looking unfavorably on the act of smoking. Makes you wonder if the Society was as much concerned about "defiling the body" as they were in looking good to the world. After all, they have frequently proclaimed their status as the largest non-smoking organization on the face of the earth. What most people don't realize, however, is the price so many JWs have had to pay in loss of family, friends, and spiritual and emotional support in order to bolster that "clean" image.