This is a much-researched question and the statistics are out there, maybe I can track down some of them via Google. In the meantime, this, from David Popenoe:
"Many studies have found that those who live together before marriage have less satisfying marriages and a considerably higher chance of eventually breaking up. One reason is that people who cohabit may be more skittish of commitment and more likely to call it quits when problems arise. But in addition, the very act of living together may lead to attitudes that make happy marriages more difficult. The findings of one recent study, for example, suggest "there may be less motivation for cohabiting partners to develop their conflict resolution and support skills." (One important exception: Cohabiting couples who are already planning to marry each other in the near future have just as good a chance at staying together as couples who don't live together before marriage).
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David Popenoe is professor of sociology at Rutgers University, where he is also co-director of the National Marriage Project and former social and behavioral sciences dean. He specializes in the study of family and community life in modern societies and is the author or editor of nine books. His most recent books are Life Without Father: Compelling New Evidence That Fatherhood and Marriage Are Indispensable for the Good of Children and Society and Promises to Keep: Decline and Renewal of Marriage in America.
This is one guy who has the statistics (in his books). I'm still trying to locate them on the Web. Just something for you to look into.
Thank you for including the part of the excerpt with the exception to marriage-planning couples. This is the point I've been trying to get across in this topic. You can't lump couples who are about to marry and intend to do so with couples who just want to "shack up"