IMO Walt's specialness is like The Shining, it can't be explained why he is that way, he just is. Maybe the others didn't even knew that Walt was special, they just kidnapped him because either he was a kid and easy to fit their ideologies in his mind, or because no baby could be born on the island. Anyway, it was until they kidnapped him that they realized he was special. They were just testing him and figuring out how they could use his abilities, until Ben used him for his plans to get the tumor removed from his back.
What do you think??