Well, after reading this I think that it's going to be impossible to decide until it is decided what kind of game this is. That of course will never be agreed on.
IMHO, It's a racing game. It is not a 'Driving Simulator', unless you regularly drive your Team Oreco Viper at 200 mph to the Piggly Wiggly for groceries or a Jaguar XJ13 down the 405 on your way to work. I'm pretty sure that's not what Kaz meant. It is not a car collecting game. I collect them, it's fun, I have painted my GT350 about 20 times now looking for the closest shade to Ford's Grabber Blue (French Blue is it, btw). But the original focus is racing, not collecting. The cars are there to try racing different types of vehicles. They're not Pokemon cards.
So the question of whether any car is not suitable for this game is kind of moot, I guess.
I think 'racing game' is a little 1-dimensional. There are so many elements of GT5 that are not strictly racing, that anybody who assumes it's a racing game will probably be ticked off at the game they received.
Everything about it seems to say 'car appreciation game', and the competition side of automobiles (racing) plays a big part, because the competition side is what makes it a game.
If you take it as a car appreciation game, just about every bit of it makes sense, even the multiple variants of the same car. The variants often show a car's evolution -- no car is 'static' in real life, it undergoes changes all the time.
Even the X2010 makes sense in this framework, the way the Nike-One made sense in GT4. Both of those cars pose the question "Here is what cars have been, and currently are -- what could be developed in the future using technologies that are either here today or on the immediate horizon?"
Also, if somebody said to you, "Choose a cross-section of cars that demonstrates the whole scope of the automobile's history", you'd have to choose some cars that have been specially adapted for different purposes, from the most basic people-movers like the 2CV, to the fastest race cars like the C9, to cars that have been adapted for water like the Schwimmwagen, to the ultra-efficient, to the most wasteful of supercars.
The game is about passion for all things automotive, IMO.