I wish more supers movies would do stuff like that. It's not necessary to see the background of every single villain that appears, nor does every one have to be a major menace (two great decisions in Dark Knight - Joker doesn't have a background, and Scarecrow got a brief cameo where Batman handed his ass to him).
Dude, you just pointed out two of my favorite elements of
The Dark Knight. When The Joker first explains how he got his scars, I was like "Lame." Then he explains it again, differently, and I realize the first story was bullshit, and
we're not going to get an origin for the Joker! That's fucking AWESOME. No connection to Batman, no origin, just this freak who came out of the woodwork.
And then the throwaway appearance of the Scarecrow. I give mega-props to Cillian Murphy for being willing to do that.
The Spider-Mythos is full of throwaway villains that they could just toss in. Shocker, Kangaroo, Rocket Racer, Prowler, White Rabbit, The Grizzly, etc. None of these characters deserve full treatment in a movie, but how cool would it be to see a scene where Mary Jane is fretting about Peter while she watches him on TV, and close observers notice that he's fighting the Vulture or the Scorpion?
Movies typically open with a scene that establishes the "ordinary," the state the protagonist finds himself in
before the story or plot begins. When writers want to show that the protagonist's normal life is one of action, the story starts with the hero doing some amazing thing. Like
The Dark Knight starts with Batman taking out Scarecrow. Irrelevant to the larger plot, but it establishes the ordinary for Batman.
Spider-Man 4 needs to start with a scene like this. After three movies, they need to establish that Spidey is a full-fledged crime fighter and that he exists in a world where not every freaking villain has some personal connection to him (though that is, of course, an important part of the Spidey mythos, as his most important enemies DO have that personal connection).