First, I simply must say that living together before marriage does NOT prepare one for living together after marriage. It's a misconception that is spread around as fact, despite there being essentially nothing to back it up.
Unless one or both of the people involved is one of those repressed nut-jobs who changes after marriage, I'm gonna have to disagree with your assessment.
You won't really know someone until you live with them.
Don't get me wrong. I'm not at all saying you'll get 100% insight into the rest of your days together, but it gives a good hint as to what one can expect. It's always a good idea to test the waters before swimming. Especially when those waters are a lifetime commitment.
I lived with my wife for 7 years before getting hitched, and the only thing marriage changed about our relationship was how close we felt to each other. We already knew what to expect out of each other at that point.
EDIT: Actual haps.
Over the weekend, I hooked up the ol' Wii for the first time in many months, so that my son and I could play some games together. When I wasn't looking, he erased my license (account) on Mario Kart Wii (by accident - he's 4). Even though I hadn't played the game in what felt like forever, I was pissed that all of my unlockables were re-locked. So, I yelled at him a bit about it, and then we made up.
We then proceeded to have a great damn time working on unlocking everything again.