Except that in an open relationship, I imagine getting to know someone you're sleeping with is probably not a good idea. Too many common interests with the bit on the side can lead to an unplanned change in relationship status.
I recently joined a poly group with 30,000 members on Fb to get more info on what it's really all about and what I've learned is that there is some of everything out there, and there's a label for everything. And there's people whose relationships are at all levels of openness from completely, to completely rules-based. And there's examples of each of these both succeeding and failing, or doing one and then the other... or vice versa.