i know some other book fans don't like the faile backstory in show, but i think it's very much in keeping with what the show's been doing in general with the shadow: with the more 3D forsaken, with liandrin, and with min's arc last season, they're showing us that darkfriends are people, who feel the same emotions as everyone else, are bitter for comprehensible human reasons, and have their vulnerabilities preyed upon by the shadow.
and part of darkfriends being people is that sometimes they have families, some of whom they're very close to. many of us love the hadnan kadere chapter, how he seemed to have genuinely cared about his family and has developed this weird cognitive dissonance around them finding him out and him having to kill them. we like ingtar, and the emotional toll it takes on rand to get to know and genuinely admire someone who turns out to be a darkfriend.
but the show doesn't have the ability to delve into the psyche of random darkfriends (or anyone) like that, and i think making a major character have a family member turn out to be a darkfriend is a really clever way to demonstrate the emotional fallout of that. and i think faile is a good choice for that; unlike with min and aviendha her parents are existing characters with significant roles, so we can have davram and/or deira show up later and continue to develop this really interesting emotional question of what you do when your family member turns out to have turned to the dark. at the same time, she's not a first-string character like elayne or the EF5, for whom making their parent a darkfriend would spawn a new emotional arc that would have to take up an absurd amount of space in the narrative.
basically the writers are giving themselves an opportunity to explore an interesting question that goes largely untouched in the book text, and they tied that opportunity to a character for whom most fans hate the late-book plotline anyway, so they have some space to play with it without it displacing a more popular plotline. it's smart writing, and i'm excited to see where they go with it eventually.