I'm torn on the UCD...
On one hand, I understand the sentiment behind finding a rare car that not many others have, and the excitement that comes with finding a rare model.
On the other hand, since turning to online play in GT5, I use GT as a driving simulator to share experiences with my friends online. I no longer have any desire to grind credits to buy cars, or to wait for a "rare" car to appear. If I have X amount of hours in a day/week/month to play the game, I want to spend all of those hours doing the things I enjoy doing (going door-to-door in online lobbies), not grinding credits, and not cycling a UCD to find a car I want to drive.
My personal solution to this dilemma would work like this:
**note, my solution is dependant on having much more customization in the game to start, in order to build personalized versions of baseline models**
I would have all baseline models available from the NCD at all times (eg, Nissan Skyline GTR R34) The UCD would be comprised of "rare" cars, ie, limited production models, tuner cars, used racecars, etc (eg, NISMO GTR R34 R-tune).
Just my take on the whole UCD issue.