I definitely agree that actively disliking a character just because she doesn't have a huge backstory doesn't make much sense, considering that if that were the case a lot of characters in most games outside of the main few should be disliked just for that fact. But it is certainly a valid justification for not giving a crap.

And I also agree that Ashe becomes the main character in the story later in the game; Vaan and Penelo both are pretty much just followers for the hell of it.