I think a lot depends on the individuals. The problem is that different people need different things at different times, and you need a lot of communication to make relationships work. Open relationships come in various forms.
Non-established relationship
Two people who see each other on a regular basis, but don't want to be exclusive with each other
This is often a trial phase, after people have met and like each other, before they have severed any rival options. Can also be a long-term option for those who feel they don't want to settle down, or at least not settle down yet. As a long-term solution it's a little less comfortable, since you're openly shopping around for someone to replace or supplement the person you're with. It can also often be that one person in relationship wants to be exclusive, but the other isn't willing to do so.
Problems: Possibly the hardest relationship to keep stable, as it's prone to drifting either to no relationship, or into an established relationship, exclusive or otherwise. In the long run it's almost impossible to lay down and maintain rules around a relationship without having one. It doesn't keep your options entirely open, since friends of your partner, or those who only want exclusive relationships for themselves, may still be unwilling to date you.
Established relationship, open to causal others.
In this two people are in an exclusive relationship, but sleep around with other people. As long as they don't form a relationship with anyone else, no problem.
This can work quite well, provided it genuinely suits both parties. Can include sharing a partner, or bring people home who you think your partner will like.
Problems: Sleeping with people without developing longer-term feelings for them is actually quite hard. Get used to continually explaining your domestic relationship to everyone you meet, or you won't get many people willing to sleep with you anyway.
Established relationship, open to long-term others
Two people in a relationship that may acquire more people that last for several years.
This can work well, but the relationship with the others is not equal. That's a serious limitation for most people. Some people, however, don't want equal place in a relationship, because they don't want to take responsibility for maintaining it. For them it can work quite well.
Problems: Inequality in the relationship can lead to stress. Adding a series of breakups to your otherwise stable relationship may not be a plus, particularly since you'll be going through your partners as well as your own.
Polyamourous relationships
Established relationships between three or more people.
This can work well, and be remarkably stable in the core, if not around the edges, particularly if everyone brings something different to the relationship. The more links between people the better, so strict heterosexuals may find themselves at a disadvantage here.
Problems: maintaining a quality relationship with another person can be a lot of work. Maintaining it with two people is more than twice as much work, because you also have to allow for their feelings for each other. As such poly relationships can involve a lot of social drama, planning, spreadsheets, and really quite large calendars. Some rely on one or two hyper-organised people who plan everything, often backed up by one or two empathic people who can reliably hug it all better. Others rely on all living in the same building and not wanting to go out much.
Are open relationships wrong? Not in themselves, no. I would say that they are likely to do more harm than good for most people, and that they are avoided for reasons which are quite practical and sensible. But if the circumstances and people are right, it can work quite well, and there's nothing wrong with it. Like all dating, however, you need to be careful or people end up getting hurt.