For the wary, a way of not making overly sure. But two recent long-term research studies independently gave evidence that living together means a given couple will be less likely to marry. And marriages thus tried aren't necessarily more true. In fact, people were more likely to stay married, and to report satisfaction with marriage, if they did not cohabit beforehand. And, researchers pointed out, the results were the same for couples with traditional and very untraditional views of marriage.
Life is hard, and it can help to have a few illusions.
And unless there is a real pathology going on, knowing someone longer doesn't necessarily mean knowing him better. Like couples of a century ago, we'd get to know each other within the context of a commitment. Precisely because the unknown was so vast, we thought we might have an edge. I couldn't say that. But I do know that the truly acquainted will have as many fiascoes as the newcomers.