I feel that "primary" and "secondary" are best used as descriptive terms, not as what a relationship "should be" before it even starts. And you can have several primaries, or have none have have secondaries only. It's about the type of relationship.
This being said, when people open up to poly after being a monogamous couple for a while, it's common for them to have a strong emphasis on the original couple. In cases like that, they might have "secondaries" who also have their own "primaries", for instance.
One might also feel they only have time or attention for one primary relationship, and want something more casual out of the others.
At any rate, I feel the relationships should reflect everyone in them. So, a more casual relationship works fine if the person you're having it with wants it to. If they want something different than what you can offer, it's not going to work so well, or it will require a lot of work and compromise.