Life will always throw bad things at you, it's just a fact, and by erasing bad memories you are erasing every life experience you ever received. No matter how bad the memory is, it taught you something, helped you grow as a person, and by erasing it you will most likely make the same mistake over again.
I believe By erasing harmful memories, we are hindering our ability to cope with life's problems, because most of life's issues are universal. 9 times out of 10 there is someone in the world who has overcome whatever issue that someone may want erased. By not allowing a person to deal with a particular issue you are weakening them, and setting them up to through the same experience all over again. The issues of life are repetitive. Lessons need to learned in order to overcome them.