In "Conversations With Dead People," which was, I believe, the last time Hank Summers was referred to, Buffy said that "He cheated." I assume that she is referring to an affair with his secretary, but there might have been several other women that we know nothing about, which would probably have been the cause of the divorce in the first place. Interestingly enough, Sarah had a bad relationship with her own father (or possibly no relationship at all. She referred to him once as "the sperm doner," not the most affectionate term to apply to one's father), and she might have been uncomfortable playing a character who loves her father. It wouldn't be the first time a star had influence on a plot line.
As an aside to Slayer Comma The...I'm truly sorry that you had less than an ideal relationship with your father, but as a father AND grandfather myself, I feel qualified to state that the vast majority of fathers do, in fact, "put their children first." I always have, my son certainly does, and every man in my family who has been blessed with children has as well.
I think it was a necessary part of the show. Giles WAS the father figure and he and Buffy treated each other as father and daughter. Looking deeper at it there seems to be an ongoing theme of poor father figures in the show, apart from giles that is, but he is nobodies father biologically.
Hank:MIA
Mr. Harris: Abusive Alcoholic
Ira Rosenberg: Little involvement in his daughters life
Mr.Chase: Replaced affection with money
Faith never knew her Father
Angel:Hated and then ate his father
Mr.Gunn: No real mention of parents but the fact that he lived on the streets says enough
Wesley: Emotionally abusive
Lorne: I can't remember his father being mentioned but I can't imagine him having a different opinion than his mother
Mr. Mc Donald: Failed his family, Lindsay hated him
No mention of Spike's father
and Angel himself, while he tried to be a great Dad, had an awful relationship with Connor
Fred is the only one with a loving relationship with her father. This is something that the show has been criticised for before, for having a rather cynical view on the child/father relationship.
As far as SMP's acting ability to play a girl with a healthy relationship with her Dad...well I don't think she would have had a problem, she is a damn fine actress and a professional. Lesser actors than her have had to face this challenge. Chad Michael Murray plays a young man abandoned by his father and has an extremely close relationship with his mother in One Tree Hill, while in real life Murray's mother left her family when he was just a boy.
On a personal note I come from a household where my father is the one to put his three daughters before ANYTHING not my mother. I also realise that this is just my experience but I agree not all Father's are Hank Summers.