Wow, lots of opinions here, OP you're going to have a hell of a job reading through all of this. Good idea too because I imagine you'll have a much, much better idea as to why by the end of this, so hats off to you for wanting to understand things from other peoples perspectives.
I for one have never smoked a cigarette in my life, it seems 100% retarded to me. Some people say that it reduces their stress levels, but I believe that's just the release of stress once 'a fix' has been received. Some people say it's social, they do it when they drink... Speaking to one another is social enough I think, copying other peoples bad habits isn't productive. Anyway, I know plenty of people would defend why they smoke to the grave, yet I don't understand at all, so I understand where you sit in all this O.P.
I have been known to drink to excess, I like the feeling of pushing my body to its limits, experiencing as much as I can in life, whether that be environments, people, or states of being. This includes getting drunk. This said, I have gone off drinking considerably in the last year or two as the positive effects it give you are minimal when they are compared to...
Drugs! Yay! I'm not a huge drug user, I binge maybe once every few months, it's by no means regular and I'm certainly not addicted to anything (yet). This weekend in fact I intend to get high on shrooms, salvia, NOS and weed. They all produce different effects, but the effects are pretty damn good (for the most part, you can get a bad trip, but the risk of getting that bad trip makes the good trips all the sweeter). As I've said before, I enjoy experiencing altered states of conciousness, the ultimate escape from (or realisation of) reality. I haven't yet tried any drugs that I believe to be dangerous, or that I think will have more adverse effects than either smoking or drinking, so to me I can't see the stigma attached to these kinds of drugs. You get amazing experiences, realisations, epiphanies, hallucinations... You could learn more about peoples experiences with drugs by browsing Erowid's experience vaults.
I'll finish off by giving you two sources which demonstrate my two main reasons why I have no problem with and enjoy illicit drugs. Here's a list from Erowid mentioning some of the positive effects of Mushrooms:
- mood lift, euphoria
- increased giggling and laughing
- creative, philosophical or deep thinking : ideas flow more easily
- boring tasks or entertainment can become more interesting or funny
- sensation of insight
- life-changing spiritual experience
- intense feelings of wonder
- paradoxical feeling of a normalcy and deep alteration of psyche
- may interrupt cluster sequences in those suffering from cluster headaches
You tell me those don't sound good

. I can't remember whether the escapist likes links or not, but here's a picture that speaks for itself: http://en.wikipedia.org/wiki/File:HarmCausedByDrugsTable.jpg
I'm 19 btw.