People spend years in psychotherapy to understand the source of their neurosis and/or emotional problems. I think that that time is well spent, though doing it in hypnotherapy saves time and money.
What is not so good is being unable to forget a painful memory. Many, if not all people, have something in their past that is excruciatingly woeful, something they regret doing or having done to them. It makes sense to go through hypnotherapy to deal with those issues. But what about the people that fully understand and have proactively processed these matters but are constantly plagued by a morbid memory which does nothing but hurt them?
This is when forgetfulness is a positive quality. If you have resolved something from your past but can’t stop thinking about it, perhaps you would benefit from hypnotherapy sessions which actually can make you forget a dismal and unnecessary recollection.