Never smoked in my life, or drank alcohol to excess. The fact is *nothing* (short of pure poison) is outright bad for you when consumed in moderation, but a lot of folks simply don't have the willpower to consume something that they love the taste or effect of in moderation (they get greedy for it/the 'hit'), and that leads to the consumption of it in excess, which is where the problems start...
Plus, when it comes to alcohol or drug abuse, that usually happens when unhappy people are trying to escape from their unhappy/unfulfilled lives, and blot it out, which is why having a 'happy head' is the best cure/antidote of all for any bad habits;)
Marco.