I think if you smoke a lot, it can have a mental effect that has quite some inertia - as opposed to booze where you are pretty much mentally clean within 48 hours or so.
Whilst the herbal cigs don't do much for me, some friends who've done regular smoking (i.e. most nights) said they started to get a bit woolly after awhile.
Personally, given the choice, I'd rather fly with a heavy drinker who'd abstained the night before, rather than a heavy toker who'd done the same.
Obviously it's all relative. I doubt the occasional smoke is going to be much trouble, kept at a suitable distance from flying.