I dont think it detracts from the story by making Scrooge less of a two dimensional character. in the original he started as a very simple 'baddie' with no hint of how he ended up there.

It may seem a leap, but I think that the early 'X Men' are made much better than many films where the villains are simply villainous. By giving Magneto a back story it changes the story from simply goodie against baddie to something much more intellectually interesting.

And something similar happened here, to my taste. Even from the earliest scenes, Scrooge was more complex as a character than most productions. to me, that enhances the story.