I think it's perfectly understandable for women not to leave bad relationships because of worries over the nasty partner being able to have the children alone for days at a time. Not saying staying for the kids is the right thing to do - just that I can see why it's difficult and not as black and white as people claim.
Leaving doesn't stop him being around the kids. He may go for 50/50 but even if he didn't, every other weekend without you there to counteract his influence is a frightening prospect.
My dad wasn't physically abusive, but he could be quite nasty especially when drunk. One time when my mum left the house for a couple of nights when I was 10. He didn't bother making me any meals. I was old enough I could make some stuff myself, so I didn't actually go hungry but I just lived off cereal and toast. He also spoke to me about how awful my mum was and strongly implied she had abandoned me forever and was not returning. I am positive he did this on purpose to hurt her.
As I look back on my childhood, I do wish my mum would have left, but now as an adult it's mostly for her benefit. The weekends I'd have had to spend with him really would have been very difficult. Having my mum shielded me from a lot of it.
If I imagine the hypothetical of her leaving when I was very young (say 5/6) and what those weekends with him would have looked like - bedtimes when I was scared of the dark, or how well I'd have eaten or how I'd have been spoken to? It's nothing social services would have been interested in, especially not back then, but it'd have been utterly miserable.
As it was, the day to day wasn't so bad. I spent most of my time with her and knew to go to bed before he got drunk. When he did say anything nasty, she defended me. I felt loved and accepted by one parent at least. It's hard for me to be angry at her for that.