It might be worth noting that some poly configurations have more than one primary relationship. For example, in my own poly family, there are two guys and one gal, and each of the men is a primary partner to the lady. So we don't have any secondary relationships, however, she and the other guy are the legally married couple, so since we are "in the closet," sometimes I have to act like "the friend."
I think usually if someone isn't happy being a secondary, the polycule will be unstable and soon break up or change somehow. I've read of a number of cases where the secondary wasn't happy. Usually it's when they were truly being given the second-class-citizen (or peon) treatment by their companions. That, I think, is why Franklin Veaux composed the Secondary's Bill of Rights, as a way to help protect against that kind of situation. If the secondary person's needs and feelings are heard and attended to, there usually isn't a problem.
I have no direct experience of being a secondary, but sometimes in the past I felt like a secondary, or like a "second-class citizen." Those were the early days, when the V's relationships were still rocky. It smoothed out after awhile.
Love means never having to say, "Put down that meat cleaver!"