Well, suppose Mr X finds a hypnotist who does remove that part of the memory, and he does indeed forget that the event happened... but all the negative emotions are still there! The memory is gone, but its aftermath stayed around. That's not really what you'd call a good result, right? What I guess is much more important is to deal with the negative emotions (basically get rid of them), and maybe removing memories needs to be part of that, or maybe not...
To quote jimmyh:
Do you think it's possible to have it not impact Mr.X's life in a negative way anymore if he doesn't forget it? Do you think that other solutions could exist, even in theory?
In my words: do you figure there could be a way for Mr X to fully remember the thing he wanted to forget, and still be able to move on and put it behind him, together with all the negative emotions he might have gotten from it until now? Because if you do, that means that there are more potential solutions to Mr X's problem, and more solutions means it's easier to find one that you can make work. Make sense?