You are right, it's not a competition. However, the OP asked if the RZ was rare. Some people are saying yes it is, while others are saying no it is not. Someone stating they have 3 or more, or whatever number, is simply them saying they don't feel it to be a rare car. I personally fall into the category of thinking it is NOT rare. I have several in my garage all with different race type specific tunes. I also keep one or two on hand for trading, and usually by it every time i see one in the UCD, as the RZ seems to always be a hot commodity, being that it is top notch for grip, drift, and drag.
I do grind the Ferrari seasonal at la sarthe on almost a daily basis, which causes my UCD to rotate very quickly, so perhaps this is why I feel they aren't rare. I think it just varies from person to person. Each individual seems to have cars that are rare for them (RUF CTR Yellowbird for me). I think I would agree with whoever said a car will refuse to show up in the UCD if you really want it. It's the video game gods toying with us lol.
What really baffles me is how not a single version of the JZA80 Supra made the premium list...mind bottling...