Besides, MJ always felt tacked on after Gwen's death. Peter needs the proper amount of time to grieve and move on. I feel like having MJ come to Peter at the end of The Amazing Spider-Man 2 (providing that Gwen... you know) would be gimmicky.
I don't know about the movie, but I think it worked fine in the comics. I don't see any way in which MJ was "tacked on," considering that she was introduced in the comics half a year before Gwen was (though Peter didn't properly meet her until 11 issues after he met Gwen). Both women were part of Peter's life, and each other's, for pretty much the entire time that Peter and Gwen were involved, and they had kind of a friendly romantic rivalry for Peter's affections, a la Betty and Veronica.
And Gwen's death is an integral part of MJ's evolution as a character. Before then, she'd always been the carefree party girl, pulling away from anything serious or heavy or meaningful. But when Gwen died and Peter angrily told MJ off for being shallow and unfeeling, her decision to stay and comfort him in his grief was a pivotal moment of growth for her, and a powerful statement of how much Peter and Gwen both meant to her. And it wasn't about her becoming the rebound babe, it was about her being there for someone who needed her friendship and support, probably the first time in her life that she'd found the strength to do that. There was nothing tacked on about it, because it wasn't just for Peter's benefit. It was the
defining moment for the MJ we know today.
Granted, it'd be hard to make that work in a movie, because it's predicated on Peter, Gwen, and MJ having that established history together. It could've fit in if the movie weren't doing so much else and had been free to focus more heavily on the romantic angle and relationship-building, but I guess that's not in the cards for a mega-budget 3D tentpole action blockbuster these days.