Hmmm. I would have agreed with that up until last year, Rhubarb. Now, I think I have a responsility to prevent my children's childhood from being blighted and punctuated by someone else's groundless, rage-filled, abusive, fantastical, protracted tantrums (they go on for months and are triggered by very, very little - think "giant overgrown two-year-old").
When they ask why we stopped seeing her when they were little, I will tell them the truth (as gently as possible and without any of the poisonous embellishments some people might add - I am not spiteful, I just want to live without the sword of Damocles over my head). I don't stop them from seeing other relatives, despite the fact that I'm not keen on some of them personally and don't much enjoy seeing them. They're family, so we make the effort. But some relatives - like my mother - fall into a whole new category. In fact I am the last in a long line of people to decide that her behaviour isn't actually tolerable any longer. Moreover, she knows perfectly well that her behaviour harms herself and those closest to her - and she knows how to get help, if she could be bothered to. It's easier to just go on living in a little bubble in which nothing ever happens unless she wants it to, and then erupt and start spraying lava everywhere if she doesn't get her way.
I get tired of people coming out with these blanket statements about maintaining family relationships being completely vital, regardless of circumstances. It's very easy for people to be holier-than-thou when they haven't got someone like that to contend with.