I actually prefer to be separate. Simply because I like to "Love the one you're with"! It's easier to concentrate and just be in the moment when I'm alone with my partner. But...Hubby prefers to watch me with another partner. So, I sometimes compromise.
It began with us talking about our fantasies and Hubby was first to bring up the swing thing. But, once I warmed up to it, I've definitely become the biggest supporter of it in our marriage!