Ah, you are repeating Ant's mantra of, "Hypnotism is a skill and therefore should be demonstratable anytime!"
The reasons for why a hypnotherapist doesn't typically work within the confines of the binary event is because our goals are too serious to risk.
Typically its a bad idea to get into an adversarial relationship with your client when you need them to follow your instructions.
Losing rapport and credibility can ultimately cost the client valuable time and money. If they start dismissing hypnosis in general you may have actually put a huge roadblock into their recovery time. All in all even if WE as the hypnotherapist fails we want to ensure that hypnosis as a tool does not get tainted.
Additionally we use a lot more information than the hypnotic process. We check diet, health habits, emotional stresses, coping mechanisms, drug use, medical history, etc.
Most hypnotherapist don't see the world like a stage hypnotist would hypnotism is a byproduct of the therapy not the other way around. Of course there are plenty of cross overs but the second you put the context of therapy into the process you have a completely different set of goals and tools to use.
Anyway my 2 cents
