I think those who live together (cohabitate) are breaking the rules of maritial conduct (and they know it). They want all the perks associated with marriage, but they both have one foot out the door.
Their mock marriages are a sham, because they are never fully invested. Each of them knows that, at any time, they can just walk out the door and leave, so it changes the dynamics of the relationship to one of just shacking up...having sex...playing house - but nothing more than that.
In tests, researchers have been able to identify cohabitating couples from dating couples and married couples. It seems those who cohabitate are more hostile to one another and lack a certain trust and closeness that the other two couples possess.
Studies have also revealed that those who live together before marriage have a 50% higher chance of getting divorced than those who do not. It seems non committal couples also develop communication barriers and show behaviors that are consistent with fear of intimacy, fear of commitment and fear of abandonment. They tend to take this baggage into subsequent relationships.
Having said that...marriage is the only way to go, but with the advent of "No Fault" divorces and the lack of negative social stigmas associated with divorce - a marital commitment is only as good as the characters of those speaking the vows. Unfortunately, in this day and age, the ink on most Marriage Licenses mean next to nothing.