I think the term "unicorn" is important for one reason: it points out the unrealistic expectations of some couples diving into what they think is poly.
I actually think we should appropriate the term for childcare! I've been looking at ads for nannies and there are so many families who want a nanny to come live in their house and do on-call nannying and housework at the family's whim, but be paid nothing except room and board. They want the person to already have a college degree and not be in school anymore. They want the person to be able to commit for years into the future. They want the person to be from their community and not just moving to the country. Etc. etc. etc.
Who would take that job? A very rare person... a unicorn. Why? Because everything about their arrangement is unfair. Nannies are people too and need established hours and duties, and, if they are doing this as a career (and not just for a place to stay while going through school, etc.), they need a living wage.
Same thing with "secondaries". They're people too, and it is a rare person indeed who would be able to accomodate every fantasy of an established couple. It's probably good that couples know that.