I was married to a woman for 10 years. But it was a bad marriage and she turned to drink. It got worse and, even though we had two young children, I left her.
But even after I left she did not stop drinking and over the years it got worse and worse. She never re-married and about 3 years ago she died.
She was only 52 and she had basically drunk herself to death.
I think there are some people who find life so bad, or so boring, or so difficult to cope with, that they blot out the pain with drink (or drugs).
We have seen many writers, pop stars, film stars etc who have died young because of their inability to manage their intake of substances.
Bredan Behan is a good example of someone who basically drunk (or drugged) themselves to death. Other include Keith Moon, Janis Joplin, Jim Morrison, and that prat going out with Kate Moss is going the same way.
As others have said, unless they want to help themsleves there is not much you can do. It is awful to watch someone waste their life like that, but some people just have this destructive streak.
Even if you left her and took the children I do not think it will snap her out of it. It is very sad.
Try to get her to watch the film "Days of wine and roses" with Jack Lemmon. This is a powerful film about how alcolholism breaks up a marriage and watching it may shock her in to realising what she is doing to herself, her marriage and her children.
More here:
http://www.imdb.com/title/tt0055895/