I disagree, in my opinion it's definitely a case of the more the merrier. One of Gran Turismo's main selling points was its vast selection of cars, and it features many car not featured in other games.
I think, when it comes to the plentiful amount of model variations with little difference (Skylines and such), it doesn't bother me. At the end of the day, it's not like it takes a lot of time away from making any other car, seeing as the car is pretty much already made save for a couple of tiny differences. And it allows people who own a certain model in real life to actually own
their own car in the game, not just "Oh, this is just like my car. But it's from two years earlier... and is more powerful... and has different bumpers". I do, however, feel they could implement these cars in a better way, something similar to
Mercedes-Benz World Racing in which all model variations of the Mercedes-Benz line-up were featured (I.e., C180, C200, C32 AMG etc.), but they were bundled together as one car in the car selection menus; and you would scroll up and down to select the model variation. Alternatively they could do something similar to Test Drive Unlimited, where you buy the basic car, and then select options afterwards. Only in Gran Turismo it would be the model variation.
I think PD would be wise to take either of these ideas into consideration for GT6. Saves having a boatload of MX-5s, Skylines, NSXs etc. clogging up the car dealerships.