Personally, I think that poly requires no justification, it is merely yet another alternative to this lifestyle. Making it work can be difficult, but I decided the best way to explain it was to draw a parallel.
One or Multiple Pets
Some people own a single pet while others own multiple pets (i.e., 3 Dogs, cats, or a mixed collection, etc.). The sole pet owner is devoted to his/her pet and has a very strong bond which is rich but usually does not think they can handle more than one pet. They conclude multiple pets cost more money, take more energy to train and demand more attention. In part, this is true while in the total scheme of things multiple pets actually demand little more incrementally.
One thing discovered is that there is less waste with multiple pets and the pets tend to complement each other in diet and activities. Each one has its own personality, each one has unique habits to show attention, and most have different skills, abilities or interests. Effectively keeping you challenged and engaged with the group. There never is lost energy with the group because one energizes the other and you get a boost through their interactions.
Walking three dogs is not three times as difficult as one, nor do you walk them serially, but as a set. It merely requires more constant control and attention that is all. One may tug more than the others, one may be distracted by flowers or marking, but they tend to want inclusion in the group. Typically, they form a unified group (a pack), and this is where their energy levels balance out. Other than the control the remaining aspects are in fact no more difficult to manage. Managing the walking of multiple dogs do require being the alpha, patient and aware of each dog's needs during the walk.
Now assume again three or more dogs, each one knows or learns (sometimes different) tricks (particularly if they come as adults). The dogs learn habits from each other, they learn to share, and they learn to work together as a team if properly managed, controlled and disciplined. Even if you add a puppy, the youngest one bonds with at least one of the older dogs while still learning from the rest. And the more long-term pets help with the discipline of the newer pets.
Now jealousy does occasionally rear its ugly head. The secret is to stamp on it immediately, discipline the instigator and separate them. Timeouts and emotional denial are by far the best punishments. If you are a strong owner, then you can Cesare your way through conflict and resolve it. After all, you are the Dominant, the Master, the one in control. Your displeasure is the one thing none of them wish to face; it is your ultimate power in these situations.
The other issue with poly is the same as introducing a new dog to the pack (your home group). You do not discuss this with them, after all, it is not their decision, but you see how they interact with each other is a social setting beforehand. You then integrate them on a trial basis. It is hard at times to decide the integration does not work, but it is always the existing members who take precedence. Essentially, you will find a true poly lover finds it hard not to continue with a new addition even if there are hurdles to overcome. For the owner let one leave after a trial is tantamount to acknowledging their failure as a Master.
Hierarchy Plays A Role
Some of the secrets to success is to ensure and maintain a hierarchy; there will always be an alpha, beta, and so on (a pecking order) among pets. There are things you can do to change the dynamic but do so with care, and as the pets, themselves readjust their position; work with it, not against it.
Now as with all poly groups, you need to make sure your pets are socially engaged not only internally but also externally. They have to be able to be walked, exercised or taken and by other owners. That owner will never replace you, but they will exert control as an extension of your own. Socializing with other pets is also necessary. It is like taking your dog to the dog parks and having them play with other pets. You love to watch the energy, joy, and pleasure they get from it. Sometimes you merely sit and watch, and at other times you join in but always mindful to demonstrate that it is your pets you preferred.
Everywhere above I said dog or pet you could have replaced it with a slave. Poly works if the Dominant makes the overt effort never to surrender that dominance.
Some of us even have a menagerie (dogs, fish, birds, and cats in any mix). Why? Well, variety. Each type has something different to offer, has different needs but in no way lessens the others in the household. Menageries are very much like a mix of masochists, service, sexual, bondage, domestic subs; all have aspects which make them fine examples of their specialty, but the mix provides the ultimate in variety and works well. You can truly enjoy watching the beautiful tropical fish, listening to birds sing, dogs playing tricks or showing affection, or cats every once in a while show their claws and need taming. It also takes the burden off of one type of pet to be all things to its master. The same is true for subs. Also, you will find there is less jealousy if the mix is sufficiently diverse that each realizes the other is no threat to their "special" relationship with their owner. What better way to experience it all and yet never burden one with expectations of the other.
So I am unapologetic about being poly. It works and can work; it takes energy (a lot of it). But it forces you to realize your full potential as a dominant, and more importantly keeps you energized through the variety it offers.