I'm under the impression that only those who want to be "hypnotized" can be. So if you want to seek therapy to stop smoking and hypnosis does the trick, good! Who cares if the hypnosis actually worked, you no longer are smoking so, don't fix what is not broken. That's my angle on the actually being helped aspect.
Now for the stage hypnosis, I don't believe in that. I think the "volunteer" gets this thought "If I don't get hypnotized then I'll make blank look bad, so I better really get under their spell." This is kinda of a "want to be hypnotized" sort of show. I'm not saying it is not fun, but I am saying the entertainment is affecting the actual results.
Now I'm not sure if this counts as hypnosis or not, but subliminal messages seem to be effective. Movie theaters would show a quick picture of popcorn so fast that people would not consciously notice. However, their subconscious would pick up on it and they would want popcorn. I do wonder if subliminal advertising would count as a form of hypnosis.
Overall I am skeptical of hypnosis but it seems like it could be probable. Everything in moderation...