As someone that has smoked [1] I think legality is important - but it's also the relatively mild effects.

If you're getting frustrated at work, you can take a break, give yourself a quick "boost" by stopping for a cig, and then go back to work. Something that's not really possible with heroin, or marijuana or any other "drug" apart from caffeine.

[1] ten years on and I still don't say "ex-smoker" as one of the mental tricks to help me deal with it

EDIT formatting

As a smoker, I wish it was illegal tomorrow. Then I could buy one or two smokes every now and then, but I would have to go well out of my way.