I'm gonna vote with slowman on this one. To suggest there could have been another 20 or 30 models is a bit absurd; consider the amount of time it would take to research and test and model a car for GT4. It's much easier to change the suspension and call it an *-spec, or even just paint the bloody thing and call it Midnight Purple, and there you have a 'new' car in the game without the enormous expence of doing a really new car.
If there were 20 fewer Skylines (or Imprezas etc) in the game there would be 20 fewer cars. So to really turn this argument on its head, we should be thankful that there are so many Skylines etc because we have more cars in the game, even if we choose not to use them.