I think it's better if the player comes up with it himself. There is a risk that Larian will write a backstory that players will not like (especially drow lolth - half will resent that the character was too cruel, half that not cruel enough, and so on), plus it's a lot of work that can be invested in companions. Don't forget that you can choose a companion as the main character at the beginning of the game! Then you will have a rich background.
But I support some differences, unique lines of dialogue and everything along those lines
