I've never been in one and I can't imagine loving someone so much to want to be in a relationship with them while at the same time being (open to sharing them with others). I suspect those who have been together for a very long time and place (compainionship & friendship) above romance, passion, and sex are the ones who may feel that "sex is just sex" and as long as their partner is there for them in other ways it's not that big of deal. Personally for me they are all equally required for me to be happy in a relationship. What makes something "special" or "valuable" is it's "rarity". Having an "exclusive relationship" means no one else has what you have with this person.
Probably the biggest benefit to "open relationships" is it eliminates the biggest "deal breaker" for most people, (Cheating). Since it's an "open relationship" those who want to see other people don't have to (lie or hide) the fact they're seeing other people. There is no "betrayal". Honesty is the most important trait to have in any kind of relationship!
You can't trust a dishonest person.