It's not limiting if they make it interesting to play, plus playing back stories will explain a whole lot that the main character is connected to the story and how he/she became the important person in the story's events.
As it turned out, I liked the origin stories in DAO. They mostly were blank slates, just with different starting points.
The danger is in defining too much of what happens before the player gets to play.
I understand that a blank character will have more freedom to play with and make your own stories, but you have no connection with the protagonist which it will be dull and unsayisfying.
I have tons of connection to the protagonist, because I defined everything about him. The dull and unsatisfying protagonist is the one I'm not allowed to control, like Shepard or Hawke.
Don't give me wrong I'm all for freedom too but it plummets storytelling of how this character came to the story. Where does the character come from? How this character become involved? What does the character used to do before he/she became an important person of the story? I think having back stories and play them is not a bad idea though only if they put more to them.
I agree that back stories are vital, but they're betterif the player gets to write them.
Those questions about what the character did before the start of the game can be answered by the player. That's the whole point of the blank slate protagonist.