I think it is not a good idea. Some couples decide to live together because they think that it would strengthen their relationship for they will get to know each other better. However, this is not true for all. Even though couples are living together as long as the thought of forever has not kicked in, then it's still as if they are two teenagers who happen to have a place to stay and can move out anytime they want.
There is something psychological into the thought of living together forever that is bound by marriage. Things change, your views about your partner will change. And this cannot be practiced no matter no long the two couples live together. Plus, there would be no legal measures that could be applied, if something bad happens, if two people aren't married.