A few days ago Comic Book Movie user "Khan" wrote an article arguing that Quicksilver and the Scarlet Witch should not be in Avengers 2. He believes that since Quicksilver and Scarlet Witch cannot be referenced as mutants in a Marvel movie because Fox has their rights that Joss Whedon would change them drastically to the point that he would be raping them like Shane Black and Drew Pearce "raped" the Mandarin character in Iron Man 3. So he says, rather than "rape" Quicksilver and Scarlet Witch, they should be left out of the Marvel Cinematic Universe completely. I would have to disagree with Khan for several reasons.
First, Joss Whedon has said that Quicksilver and Scarlet Witch would keep their powers because they are "visually interesting." All Joss would have to do is somehow explain that Scarlet Witch and Quicksilver got their powers a little differently than being mutants. Changing their origin does not change who they are. Just ask the Joker from The Dark Knight for proof of that. This would mean that no "raping" would occur.
Second, the introduction of Quicksilver and Scarlet Witch means that Cap's Kooky Quartet would be born in the movies. For those who dont know Cap's Kooky Quartet occurred in the comics when Iron Man, Thor, Ant-Man, and Wasp left the team temporarily because they needed a break. Thus Quicksilver, Scarlet Witch, and Hawkeye became members of the team. Now if the Hulk returns and Ant-Man and Wasp are also introduced then two birds would be killed with one stone with the original members from the comics coming to the movies and Cap's Kooky Quartet.
Third, the introduction of Scarlet Witch could inspire the plot for another Avengers movie down the road providing there are Avengers movies after the third one. Instead of fighting supervillains in Avengers 4,5 or 6 the Scarlet Witch could go insane and disassemble the Avengers like she did in the comics with one of the Avengers most famous storylines. And if Marvel gets the Xmen rights back from Fox then House of M could be a plot for another Avengers movie as well.
In essence what I am saying with all of this is that the introduction of Scarlet Witch and Quicksilver in Avengers 2 has opened up a whole world of possibilities for Marvel movies in the years to come. Therefore it goes to show that Joss Whedon has made another great choice for his Avengers franchise.