And I don't care about the end catharsis, if the struggle is one that I can't accept that the character should have been stupid and weak and OOC enough to get into in the first place.
I think the point is to humanize the character by showing that they are capable of "weakness" or not using his/her intelligence, and by putting them in circumstances in which they are more susceptible to bad things around them, or making bad decisions.
Whether that's good storytelling or not is subjective opinion, but I'm pretty sure that's the theory behind this kind of thing.
I'm not familiar with Spider-man or the Dark Phoenix Saga (beyond the 90s cartoon version, that is! heh), but I think it's kind of the same reasoning that brought about Identity Crisis. "Let's put our virtuous heroes into a situation where they think the right thing is mind-wiping Batman and giving Doctor Light a magical lobotomy," that'll be dramatic. (The fridging of Sue was a horrible side effect of that premise, IMO.)