Cohabitation

I had a conversation with a coworker today in which she was telling me about the negative experience of living with an old boyfriend. She said she lived with him for over 3 years, and that she ended up leaving him because they turned out to be completely incompatible. Then she ended the thought by saying something that stuck in my head: "That's why I'll never live with a man again until I'm married."

I admit that it's counter-intuitive, but couldn't you look at that entire experience and say that the system worked perfectly for her? To me, the major purpose of moving in with a girlfriend/boyfriend is to determine whether or not a relationship can work through the day to day issues of life as a married couple. A dry-run, if you will. The failure of a relationship to survive cohabitation is as sure a sign as any that it would also not survive marriage. Although surely painful, wouldn't we all want such a sign as early as possible? Of course we would all like for things to work out, but why is it that some consider it a mistake when it doesn't?