My bf and I discussed it at several points in the relationship. At the time our reasonings were that this way we wouldn't have to find random roommates and/or we'd save money over renting by ourselves. Also we'd been dating for several years and thought we could handle it. It was never a way to take our relationship to the "next" level or anything like that.
However we decided against it for several reasons, the Catholicism thing makes both of our families against it (for the most part, my mom is probably the most flexible as far as that goes) and honestly has an effect on our own opinions as well. Plus I'm the messy one in the relationship and I'm still working on getting a handle on that to where he's comfortable with it. He doesn't want to clean up after me, and I don't blame him
So in the end we decided not to, I'm not sure we never will, but just like we're not ready to get married yet (though I think we will be eventually) we're not ready for that step now.
Not really an opinion either way, but there you go. I did have a classmate recently try and convince me that not moving in was just me living my life based on my parents' rules... because I mentioned they were against it as well. It was sort of odd the way she did it, but she's also almost 10 years older than I am and I don't think she took that into consideration.