I don't think punishment, no matter how you define it, should play a role in therapy. I'm an adult and do not need to be punished by someone else (I do enough self-punishment as it is). I think that therapy should be built on mutual respect and understanding. Punishment kind of implies a power imbalance with the therapist holding power over the client. Personally, I would never work with a therapist that wanted to hold power over me.

I understand the use of punishment in operant conditioning and even with that definition - something added or removed which makes a behavior less likely to occur - I just don't see how it could be effective in therapy. I think that a therapist pointing out potential consequences can be helpful and respectful. A therapist taking it upon themselves to add an aversive or remove something of value to make a behavior less likely to occur has a huge potential to backfire spectacularly when dealing with someone with a mental illness.