Just to add to my post above:
Several people have said something to the effect that if you wiped bad memories, your coping skills/mechanisms for future similar bad experiences would be worse/diminished/non-existent (hope I'm not misquoting those of you who said something like that.)
In a way I agree - it undoubtedly applies in some situations. However...probably due to being rather over-sensitive/a bit neurotic, and due to Ni habits of relating everything to everything else...what often happens to me is that if I have a very negative experience, it brings back all the pain and unpleasantness of the previous similar experiences, and it's sort of like I'm reacting again to ALL those experiences, not just the current one. Which can be overwhelming and hideously painful and probably explains why there are certain types of negative experiences which just flatten me and take an exceptionally long time for me to recover from.
I would tend to put those sorts of memories/experiences into the "hit the erase button, because I learned something but the damage wasn't worth it" category.