I don’t use it in conversation, but I admit I think about it, particularly in terms of what I am able to handle at a given time — I’m a one-primary-sort-of-person and I’m in grad school, so my bandwidth is really limited and I’m extremely cautious of overcommitting. Anyone I get involved without outside my partner needs to be well-supported in their life overall, because I cannot count on being able to provide that for them.
My husband does not use those terms, or at least didn’t when last we talked about it. He is very much a “let things find their own level” sort of person, and if that meant he was deeply compatible and entangled with multiple people at the same time on whatever levels worked, the more the merrier.
His girlfriend very much uses primary/secondary terminology, at least in part to manage her own expectations of behavior and access for herself and my husband.