Regarding the scene with Mary Jane talking to her friend: the purpose of the scene wasn't to reveal MJ's reasons for marrying John. We already know what, to a large extent, those reasons are. The purpose was to add another layer to her already-established motivations. In other words, when MJ says "My father always said, 'You'll never be worth anything. No man will ever want you'", she isn't giving a justification for a legitimate life decision; she's reflecting some broader insecurity.
In Spider-Man 1, we witnessed that verbal abuse; Mr. Watson screams at MJ, "You're trash, you'll always be trash, just like her!" as the latter escapes to the security of high-school faces. (I can't imagine overhearing your boyfriend's father repeating the same slur later would do much for your self-esteem, either) It is in this vulnerable state that the gentle charm of one Peter Parker is first impressed upon her. "You're the only man who's ever been there for me," she tells him.
In Spider-Man 2, Mary Jane makes some reference to her father coming to see her show to callously ask her for money (ostensibly to fuel his alcoholism).
In Spider-Man 3, when MJ reads her bad reviews, she admits, "When I read these reviews, it's like my father wrote them". There was even (reportedly) to have been a scene later in the film where Peter, under the influence of... you-know-what, confronted Mary Jane's father once and for all and sort of roughed him up.
So this is obviously a deep-seated insecurity that, true to the comics, MJ struggles with over a length of time; she will likely always struggle with feelings of self-worth. It wasn't a reason invented for the sole purpose of "explaining" MJ's decision to accept John's proposal.
Anyways, the point of the scene, the way I saw it anyway, was to cement the idea that Mary Jane doesn't really love this guy; she is merely appealed by what he represents. Security; a father-figure; a provider. Not an "empty seat", like Peter Parker. John Jameson is Mr. Right Now.
In this context, the scene is a welcome addition and provided some much-needed characterization to Ms. Dunst's otherwise distant Mary Jane.
I'm new here, by the way. You can call me Jason.