I always think of primary and secondary as descriptive terms. I can't imagine trying to impose things on relationships, they just evolve on their own and you deal with it. I can't just put constraints on them, I mean that's why monogamy doesn't work for me to begin with, because "thou shalt not fall in love again" is assumed.
Same thing here, "thy relationship shalt not evolve" is just as absurd to me, I can't control that.
So having several primaries and no secondaries, or several secondaries and no primaries, are for me just a description of your relationship to help people. In mono relationships, usually you say things like "we're exclusive now" or "we got a house" or "we're married" and that's a brief way to explain what kind of stage your relationship is in.
In poly, that doesn't work as well, except maybe the house part. I guess you could say you live together and people would have an idea of the kind of relationship you have, but still there is so much variety.... People came up with primary and secondary so that in one word you can give an idea of what stage you're in.
Keep in mind I don't see stages as things that follow one another or have to be in a specific order.