I think Shonda Rhimes was mainly going for my segment: individuals who have gone through hard times such as homeless, addiction or abandonment issues. While I don't condone any of Anna's methods, she felt abandoned her entire life, and had trouble seing how her bad behavior would hurt others.
The mini-series is long, as we see into the emotions/reactions of all the key players.
So some who watched it might get frustrated, but a lot of us "felt" what all went through, including Rachel, etc.