I've quickly compiled
a list of the "cr --" and "hidden" cars in GT3 and GT4. I whipped this up pretty quickly, it may be missing "hidden" cars. ("Hidden" cars are the cars that do not appear at all in any dealerships and are acquired as prize cars only)
In GT3, things looked rather good. The vast majority of the "normal cars" were available for purchase. All of the "cr --" cars were concepts, prototypes or race cars. The "hidden" cars comprised all tuner cars, some of the real race cars and all of the fantasy race cars. Only three "normal" cars are in this group, the very rare R390 GT1 and GT40 and the not as rare Mini Cooper. IMO, GT3 is closer to the ideal model I'd like to see.
In GT4, things are different. With the return of used cars to the series, certain cars are "cr --" in the new car dealerships but also appear in the used car dealerships and can be purchased there. All concept cars appear as "cr --", however there is more variability with everything else. As in GT3, certain race cars appear as "cr --" and some tuner cars are "cr --", but certain "normal" production cars also can not be purchased, mostly of the older (pre-90s) and low-production varieties. The "hidden" cars of GT4 are predominantly fantasy race cars.
Personally, I prefer the way GT3 did things, GT4 has far too many "normal" older cars as "cr --", they went overboard, but I do think certain low-production cars (such as the RUF CTR, Impreza 22B, M3 GTR, CLK DTM AMG, etc.) would make great exclusive prize cars.
Not fun playing a game... It must have been terrible having to complete some of the game to get cars you want to drive. My sympathies.
I don't think you want GT to be a "game", but instead a simulation tool (at least the offline portion of it). Fortunately, I think PD realizes that GT needs to have some semblance of a "game" to be relevant in the mass market.