I think we can see from the opinions expressed here that for polyamorists, non-hierarchical paradigms work best. Western culture, the US Constitution, etc., place emphasis on equality, so I believe that carries over into what we (most of us) want in polyamorous relationships.
The idea of the sacred couple "adding a third" as an accessory, a fun plaything, a fuck toy, an "enhancement" to their "real" relationship was always insulting to the "third." It might come from swinger culture, where the MF couple is the sort of foundation, and everyone else they "play" with is less than fully human, just used for their looks, charm and sexual parts. You don't need to know their history, their joys, certainly not their sorrows or their struggles. Let's just "have fun-- drink, dance, hit a bowl, eat a snack, flirt, have adult fun, fuck," and say goodnight.
You can become infatuated with a person you are just "having fun" with. But in swinging, "real life" happens with your opposite sex partner, the commitment, jobs, mortgage, home renovations, maintenance, kids and their education, lovely vacations and holidays, and so on.
So, as a mere "secondary," you could be seen as the "good time girl/boy," nothing serious, a leisure-time activity, so to speak. And if that's all you seek as you seek a secondary, or if that's all you want as a secondary, well and good. But if you acknowledge that people tend to fall in love and want to share more than the "good times," have a "ride or die," and not just a "fair weather friend," it's probably best to stay away from dating people in hierarchical relationships. If you're sick, or injured, or your dog or your mom is dying, you won't be able to rely on a person who puts their "real" partner ahead of you, always.