What are the pros and cons of moving in before marriage?

My mother has always said "why would he buy the cow when he gets the milk for free?" She's referring to pre-marital sex, but I think it applies to this situation too. Why bother getting married once you already live together? A new marriage should be fun and exciting (as well as a lot of work). If you live together, then you get married, its not like anything has really changed (other than your name). Its like your ruining marriage before you even marry!