First, I'll say they're bad, and then I'll just say that they're not relationships. If you're single, you can be open to all the options that are out there for you, but when you decide to be in a relationship, that changes. Relationship means that person is different from all your other "options" out there. If you don't feel that way about them, don't call it a relationship.
And for the few people, I've seen declare that they were in an open relationship, it didn't last long because one of them eventually catches stronger feelings than the other which causes an emotional meltdown in the long run. They begin to feel hurt that their partner could find happiness with people other than themselves. So moral of the story...just don't get into an open relationship or marriage.