Its definitely true that there are some memories that people have no benefit from holding on to; memories of war, rape, and general trauma can often be very damaging while providing little insight into life. However, the vast majority of painful memories people have can teach them something, and people forgetting so easily really does doom them to repeat their behavior.
No, the use of propanolol will not cause the person to continue to make choices that result in the painful experience, because although the person will feel less pain in the future, they will still remember having needed the propanolol and having suffered the painful experience. Having gone through the experience will still make the person risk-averse in the future. Also, many experiences are not intentional, or caused by the person's own behavior or choices.