Originally Posted by
Mister Mets
In another thread, I was asked why I'm okay with One More Day, but not Parallel Lives.
Whatever your opinion of Parallel Lives, it changed the context of quite a few scenes when Mary Jane knew that Peter Parker was Spider-Man in every interaction she had with him, or with a member of his supporting cast. You might think the new context is more interesting, but it wasn't intended by the original writers. It suggests that earlier scenes could not be taken at face value.
Parallel Lives makes it unambiguously clear that MJ learned on the night Uncle Ben was killed, which means she was aware of his secret in every appearance.
MJ knowing that Peter Parker was Spider-Man changes the broad strokes of their relationship. MJ knowing that Peter Parker was Spider-Man affects why she chose to hang out with him. In every interaction, she knows that he's really a famous superhero.
One More Day doesn't pretend that everything in older comics happened exactly the way it did on the page post-retcon. It suggests the broad strokes are the same, but that some conversations could have gone differently, etc. Parallel Lives suggests that every scene with Peter and MJ from Amazing Spider-Man #42-257 occurred with MJ knowing his secret identity, even if it doesn't make a whole lot of sense in every single scene that she would act a particular way knowing that Peter Parker was Spider-Man.
There is also the question of tradeoffs. One More Day erased the marriage and made Peter single again. We could debate about whether it was a good move (and we have. A lot. So so often.) but it was consequential. The revelation that Mary Jane knew that Peter was Spider-Man since before her first appearance has rarely been referenced, so there's very little gain in retroactively making her act like a weirdo in earlier appearances. I'm not going to disagree that One More Day retcons a lot of comics, including some that are good and well-regarded. If you think the marriage wasn't the best long-term approach for the comics, and that killing off MJ or having her and Peter divorce weren't good options, the only way to undo the marriage was through some kind of massive retcon. It changed the books going forward.
Parallel Lives didn't have much of an effect on the books going forward. Hell, it was widely considered to be out of continuity until Busiek referenced it in Untold Tales of Spider-Man. We have also never seen anyone explore Mary Jane's perspective from the issues affected by Parallel Lives (IE- what it was like to be a supporting character who knew Peter's secret).